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THE  miERNATIONAL  SCIENTIFIC  SERIES. 
VOLUME  XXVI. 


/ Crimson  ZaJce. 
2.  Vermilion. 

3 Chrome^ 
i/ellmv. 


4.  Green  . 

5. CvbaZCBliie. 
6Jl//r^ZfJClu^. 


mci: 

Blue. 


Braic 


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Greece. 

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BffeetB  pwefumi  dyi/ucrinppipmivit^^^ 


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THE  INTERNATIONAL  SCIENTIFIC  SERIES. 


MODERN 

CHROMATICS 

WITH  APPLICATIONS  TO 

ART  AND  INDUSTRY. 


BY 

OGDE^sT  N.  KOOD, 

PROFESSOR  OP  PHYSICS  IN  COLUMBIA  COLLEGE. 


WITH  ISO  ORIGINAL  ILLUSTRATIONS, 


NTEW  YORK: 

D.  APPLETON  AND  COMPANY, 

549  AND  551  BROADWAY, 

1879. 


OOPTEIGHT  BY 

D.  APPLETON  & COMPANY, 
1879. 


TO 

De.  WOLCOTT  GIBBS 

THIS  VOLUME  IS  INSCKIBED, 

AS  A SMALL  MAKE  OF  THE  ATTACHMENT 
AND  ADMIRATION  OF 


THE  AUTHOR. 


PEE;F  AOE, 


It  was  not  my  intention  to  write  a preface  to  this 
book,  as  I have  nsually  fonnd  such  compositions  neither 
instructive  nor  amusing.  On  presenting  the  manuscript 
to  my  publishers,  however,  it  was  suggested  that,  al- 
though prefaces  are  of  no  particular  use  to  readers,  yet 
from  a certain  point  of  view  they  are  not  without  value. 

I accordingly  beg  leave  to  state  that  my  object  in  this 
work  has  been  to  present,  in  a clear,  logical,  and  if  possi- 
ble attractive  form,  the  fundamental  facts  connected  with 
our  perception  of  colour,  so  far  as  they  are  at  present 
known,  or  concern  the  general  or  artistic  reader.  For 
the  explanation  of  these  facts,  the  theory  of  Thomas 
Young,  as  modified  and  set  forth  by  Helmholtz  and 
Maxwell,  has  been  consistently  adhered  to.  The  whole 
class  of  musical  theories,  as  well  as  that  of  Field,  have 
been  discarded,  for  reasons  that  are  set  forth  in  the  text. 

Turning  now  from  the  more  purely  scientific  to  the 
aesthetic  side  of  the  subject,  I will  add  that  it  has  been 
my  endeavour  also,  to  present  in  a simple  and  comprehen- 
sible manner  the  underlying  facts  upon  which  the  artistic 
use  of  colour  necessarily  depends.  The  possession  of  these 


VI 


PREFACE. 


facts  will  not  enable  people  to  become  artists ; bnt  it  may 
to  some  extent  prevent  ordinary  persons,  critics,  and  even 
painters,  from  talking  and  writing  about  colour  in  a loose, 
inaccurate,  and  not  always  rational  manner.  More  than 
this  is  true : a real  knowledge  of  elementary  facts  often 
serves  to  warn  students  of  the  presence  of  difficulties 
that  are  almost  insurmountable,  or,  when  they  are  already 
in  trouble,  points  out  to  them  its  probable  nature ; in 
short,  a certain  amount  of  rudimentary  information 
tends  to  save  useless  labour.  Those  persons,  therefore, 
who  are  really  interested  in  this  subject  are  urged  to 
repeat  for  themselves  the  various  experiments  indicated 
in  the  text. 

In  the  execution  of  this  work  it  was  soon  found  that 
many  important  gaps  remained  to  be  filled,  and  much 
time  has  been  consumed  in  original  researches  and  ex- 
periments. The  results  have  been  briefly  indicated  in 
the  text ; the  exact  means  employed  in  obtaining  them 
will  be  given  hereafter  in  one  of  the  scientific  journals. 

To  the  above  I may  perhaps  be  allowed  to  add,  that 
during  the  last  twenty  years  I have  enjoyed  the  great 
privilege  of  familiar  intercourse  with  artists,  and  during 
that  period  have  devoted  a good  deal  of  leisure  time  to 
the  practical  study  of  drawing  and  painting. 


0.  hi.  E. 


CONTENTS 


CHAPTER  I. 

PAGE 

Transmission  and  Eeflection  of  Light,  . . . . . 9 

CHAPTER  II. 

Production  of  Colour  by  Dispersion,  . . . . . .17 

CHAPTER  III. 

Constants  of  Colour,  ........  30 

CHAPTER  IV. 

Production  of  Colour  by  Interference  and  Polarization,  . . 43 

CHAPTER  V. 

Colours  of  Opalescent  Media,  . . . . . . . 53 

CHAPTER  VI. 

Production  of  Colour  by  Fluorescence  and  Phosphorescence,  . 62 

CHAPTER  VII. 

Production  of  Colour  by.  Absorption,  . ...  66 

CHAPTER  VIII. 

Abnormal  Perception  of  Colour  and  Colour-Blindness,  . . 92 

CHAPTER  IX. 

Young’s  Theory  of  Colour, 108 


viii 

CONTENTS. 

Mixture  of  Colours,  . 

CHAPTER  X. 

PAGE 

Complementary  Colours, 

CHAPTER  XI. 

. . 161 

CHAPTER  XII. 

Effects  produced  on  Colour  by  a Change  in  Luminosity  and  by 

MIXING  IT  WITH  WhITE  LiGHT, 181 

CHAPTER  XIII. 

Duration  of  the  Impression  on  the  Retina,  ....  202 

CHAPTER  XIV. 

Modes  of  arranging  Colours  in  Systems,  . . ■ . . 209 


Contrast, 

CHAPTER  XV. 

235 

CHAPTER  XVI. 

The  Small  Interval  and  Gradation, 278 

CHAPTER  XVII. 

Combinations'  of  Colours  in  Pairs  and  Triads,  ....  286 

CHAPTER  XVIII. 

Painting  and  Decoration, 305 

Note  on  two  recent  Theories  of  Colour, 324 


Index, 


326 


MODERN  CHROMATICS, 


CHAPTER  I. 

THE  REFLECTION  AND  TRANSMISSION  OF  LIGHT. 

As  long  ago  as  1795  it  occurred  to  a German  physicist 
to  subject  the  optic  nerve  of  the  living  eye  to  the  influence 
of  the  newly  discovered  voltaic  current.  The  result  obtained 
was  curious  : the  operation  did  not  cause  pain,  as  might 
have  been  expected,  but  a bright  flash  of  light  seemed  to 
pass  before  the  eye.  This  remarkable  experiment  has  since 
that  time  been  repeated  in  a great  variety  of  ways,  and 
with  the  help  of  the  more  efficient  electric  batteries  of  mod- 
ern times;  and  not  only  has  the  original  result  of  .Pfaff  been 
obtained,  but  bright  red,  green,  or  violet,  and  other  hues 
have  been  noticed  by  a number  of  distinguished  physicists. 
If,  instead  of  using  the  electrical  current,  mechanical  force 
be  employed,  that  is,  if  pressure  be  exerted  on  the  living 
eye,  the  optic  nerve  is  again  stimulated,  and  a series  of  bril- 
liant, changing,  fantastic  figures  seem  to  pass  before  the 
experimenter.  All  these  appearances  are  distinctly  visible 
in  a perfectly  dark  room,  and  prove  that  the  sense  of  vision 
can  be  excited  without  the  presence  of  light,  the  essential 
point  being  merely  the  stimulation  of  the  optic  nerve.  In 
the  great  majority  of  instances,  however,  the  stimulation  of 
the  optic  nerve  is  brought  about,  directly  or  indirectly,  by  the 


10 


MODERN  CHROMATICS. 


aid  of  light ; and  in  the  present  work  it  is  principally  with 
vision  produced  in  this  normal  manner  that  we  have  to  deal. 

Back  in  the  rear  portion  of  the  eye  there  is  spread  out  a 
delicate,  highly  complicated  tissue,  consisting  of  a wonder- 
fully fine  network  woven  of  minute  blood-vessels  and 
nerves,  and  interspersed  with  vast  numbers  of  tiny  atoms, 
which  under  the  microscope  look  like  little  rods  and  cones. 
This  is  the  retina  ; its  marvellous  tissue  is  in  some  mysteri- 
ous manner  capable  of  being  acted  on  by  light,  and  it  is 
from  its  substance  that  those  nerve-signals  are  transmitted  , 
to  the  brain  which  awake  in  us  the  sensation  of  vision. 
For  the  sake  of  brevity,  the  interior  globular  surface  of  the 
retina  is  ordinarily  called  the  seat  of  vision.  An  eye  pro-  ^ 
vided  only  with  a retina  would  still  have  the  capacity  for  ] 
a certain  kind  of  vision  ; if  plunged  in  a beam  of  red  or  | 
green  light,  for  example,  these  colour-sensations  would  be  I 
excited,  and  some  idea  might  be  formed  of  the  intensity  or  ] 
purity  of  the  original  hues.  Some  of  the  lower  animals 
seem  to  be  endowed  only  with  this  rudimentary  form  of  [ 
vision  ; thus  it  has  lately  been  ascertained  by  Bert  that 
minute  crustaceans  are  sensitive  to  the  same  colours  of  the  i 
spectrum  which  affect  the  eye  of  man,  and,  as  is  the  case 
with  him,  the  maximum  effect  is  produced  by  the  yellow 
rays.  With  an  eye  constructed  in  this  simple  manner  it 
would,  however,  be  impossible  to  distinguish  the  forms  of  ex- 
ternal objects,  and  usually  not  even  their  colours.  AVe  have, 
therefore,  a set  of  lenses  placed  in  front  of  the  retina,  and 
so  contrived  as  to  cast  upon  it  very  delicate  and  perfect 
pictures  of  objects  toward  which  the  eye  is  directed  ; these 
pictures  are  coloured  and  shaded,  so  as  exactly  to  match  the 
objects  from  which  they  came,  and  it  is  by  their  action  on 
the  retina  that  we  see.  These  retinal  pictures  are,  as  it 
were,  mosaics,  made  up  of  an  infinite  number  of  points  of 
light  ; they  vanish  with  the  objects  producing  them — 
though,  as  we  shall  see,  their  effect  lasts  a little  while  after 
they  themselves  have  disappeared. 


THE  REFLECTION  AND  TRANSMISSION  OF  LIGHT.  H 


This  leads  ns  in  the  next  place  to  ask,  What  is  light, 
that  agent  which  is  able  to  produce  effects  which  to  a 
thoughtful  mind  must  always  remain  wonderful  ? ” A per- 
fectly true  answer  to  this  question  is,  that  light  is  some- 
thing which  comes  from  the  luminous  body  to  us  ; in  the 
act  of  vision  we  are  essentially  passive,  and  not  engaged  in 
shooting  out  toward  the  object  long,  delicate  feelers,  as  was 
supposed  by  the  ancients.  This  something  was  considered 
by  Sir  Isaac  Newton  to  consist  of  fine  atoms,  too  fine  al- 
most to  think  of,  but  moving  at  the  rate  of  186,000  miles  in 
a second.  According  to  the  undulatory  theory,  however, 
light  consists  not  of  matter  shot  toward  us,  but  of  undula- 
tions or  waves,  which  reach  our  eyes  somewhat  in  the  same 
way  as  the  waves  of  water  beat  on  a rocky  coast. 

The  atoms,  then,  which  compose  a candle  flame  are 
themselves  in  vibration,  and,  communicating  this  vibratory 
movement  to  other  particles  with  which  they  are  in  con- 
tact, generate  waves,  which  travel  out  in  all  directions,  like 
the  circular  waves  from  a stone  dropped  into  quiet  water ; 
these  waves  break  finally  upon  the  surface  of  the  retina, 
and  cause  in  some  unexplained  way  the  sensation  of  sight — 
we  see  the  candle  flame.  Substances  which  are  not  self- 
luminous  cannot  be  seen  directly  or  without  help  ; to  ob- 
tain vision  of  them  it  is  necessary  that  a self-luminous  body 
also  should  be  present.  The  candle  flame  pours  out  its 
flood  of  tiny  waves  on  the  objects  in  the  room  ; in  the  act 
of  striking  on  them  some  of  the  waves  are  destroyed,  but 
others  rebound  and  reach  the  eye,  having  suffered  certain 
changes  of  which  we  shall  speak  hereafter. 

This  rebound  of  the  wave  we  call  reflection  ; all  bodies 
in  the  room  reflect  some  of  the  candle  light.  Surfaces  which 
are  polished  alter  the  direction  of  the  waves  of  light  falling 
on  them,  but  they  do  not  to  any  great  extent  scatter  them 
irregularly,  or  in  all  directions.  It  hence  follows  that  pol- 
ished surfaces,  when  they  reflect  light,  present  appearances 


12 


MODERN  CHROMATICS. 


totally  unlike  those  furnished  by  surfaces  which,  though 
smooth,  are  yet  destitute  of  polish  ; the  former  are  apt  to 
reflect  very  much  or  very  little  light,  according  to  their 
positions,  but  this  is  not  true  to  the  same  extent  with  un- 
polished surfaces.  The  power  which  different  substances 
have  under  various  circumstances  to  reflect  light  is  not 
without  interest  for  us  ; we  shall  see  hereafter  that  this  is 
a means  often  employed  by  nature  in  modifying  colour. 

As  a general  thing  polished  metallic  surfaces  are  the 
best  reflectors  of  light,  and  may  for  the  most  part  be  con-  , 
sidered  by  the  artist  as  reflecting  all  the  light  falling  on 
them.  Polished  silver  actually  does  reflect  ninety-two  per 
cent,  of  the  light  falling  perpendicularly  on  it ; and  though 
the  percentages  reflected  by  steel  and  other  metals  are 
smaller,  yet  the  difference  is  not  ordinarily  and  easily  dis- 
tinguished by  an  untrained  eye. 

The  case  is  somewhat  different  with  smooth  water  : if 
light  falls  on  it  making  a small  angle  with  its  surface,  the 
amount  reflected  is  as  large  as  that  from  a metallic  surface  ; 
while,  if  the  light  falls  perpendicularly  on  it,  less  than  four 
per  cent,  is  retlected.  Thus  with  a clear  blue  sky  and 
smooth  water  we  find  that  distant  portions  of  its  surface 
appear  very  bright,  while  those  at  the  feet  of  the  observer 
are  of  an  almost  unbelievable  dark-blue  tint.  In  this  par- 
ticular instance,  the  difference  between  the  brightness  of 
near  and  distant  portions  of  the  water  is  still  further  exag- 
gerated by  the  circumstance  that  the  sky  overhead  is  less  ^ 
luminous  than  that  near  the  horizon  ; and  the  distant  por-  | 
tions  of  the  sheet  of  water  reflect  light  which  comes  from  I 
the  horizon,  the  nearer  portions  that  which  has  its  origin  . 
overhead.  The  reflecting  power  of  water  is  constantly  * 
used  by  artists  as  a most  admirable  means  of  duplicating  in  i 
a picture  a chromatic  composition,  and  easily  affords  an  op- 
portunity, by  slight  disturbances  of  its  surface,  for  the 
introduction  of  variations  on  the  original  chromatic  design. 

It  may  here  be  remarked  that  in  actual  landscapes  con- 


THE  REFLECTION  AND  TRANSMISSION  OF  LIGHT.  13 


taining  surfaces  of  still  water,  it  ordinarily  happens  that 
the.  reflected  pictures  are  not  exactly  identical  with  those 
which  are  seen  directly,  and  the  difference  may  often  he 
considerable.  For  example,  it  may  easily  be  the  case  that 
an  object  beyond  the  water,  and  situated  at  some  distance 
from  it,  is  not  seen  in  the  reflected  picture  at  all,  light  from 
it  either  not  reaching  the  water,  or  reaching  the  water  and 
not  being  reflected  to  the  eye  of  the  observer. 

Polished  surfaces,  as  we  have  seen,  reflect  light  not  only 
in  large  quantity,  but  they  as  it  were  press  the  light  well 
together  in  rather  sharply  defined  masses  ; with  unpolished 
surfaces  the  case  is  entirely  different,  the  light  which  falls 
on  them  being  scattered  in  all  directions.  Hence,  where- 
ever  the  eye  is  placed,  it  receives  some  of  this  light,  and  a 
change  of  position  produces  far  less  effect  on  the  quantity 
received  than  is  the  case  with  light  reflected  from  polished 
surfaces.  Owing  to  their  power  of  scattering  light  in  all 
directions,  rough  surfaces,  however  situated,  never  send 
very  intense  light  to  the  eye. 

If  a surface  of  white  linen  drapery  be  illuminated  by  a 
dozen  different  sources,  it  will  reflect  to  the  eye  a sample  of 
each  kind  of  light,  and  what  we  call  its  hue  will  be  made 
up  of  as  many  constituents.  When  we  remember  that  all 
the  different  objects  in  a room  reflect  some,  and  usually 
coloured  light,  we  see  that  the  final  tint  of  our  piece  of  linen 
drapery  depends  not  only  on  the  circumstance  that  its  natu- 
ral colour  is  white,  but  also  on  the  presence  and  proximity  of 
curtains,  books,  chairs,  and  a great  variety  of  objects  ; the 
final  colour  will  hence  not  be  exactly  white,  but  some  delicate, 
indescribable  hue,  difficult  of  imitation  except  by  practiced 
artists.  With  objects  which  are  naturally  coloured,  or  which 
show  colour  when  placed  in  white  light,  the  case  becomes 
still  more  complicated.  Let  us  suppose  that  our  drapery 
when  placed  in  pure  white  light  appears  red  ; its  hue  will 


14 


MODERN  CHROMATICS. 


still  be  modified  by  the  light  it  receives  from  objects  in  the 
room : for  example,  if  it  receives  some  green  light  from 
objects  of  this  colour  placed  in  its  neighbourhood,  the  red 
hue  will  incline  toward  orange ; if  the  added  portion  of 
light  be  yellow,  the  tendency  to  orange  will  be  still  more 
marked ; on  the  other  hand,  light  received  from  blue  or 
violet  surfaces  will  cause  the  red  to  pass  into  crimson  or 
even  purple.  The  grandest  illustrations  of  these  changes 
we  find  in  those  cases  where  objects  are  illuminated  simul- 
taneously by  the  yellow  rays  of  the  sun  and  the  blue  light 
of  the  clear  sky:  here,  by -this  cause  alone,  the  natural 
colours  of  objects  are  modified  to  a wonderful  extent,  and 
effects  of  magical  beauty  produced,  which  by  their  intricacy 
almost  defy  analysis.  The  nature  of  these  changes  will  be 
considered  in  a subsequent  chapter,  after  the  principles  upon 
which  they  depend  have  been  examined. 

Finally,  it  may  not  be  altogether  out  of  place  to  add 
that  the  majority  of  paintings  and  chromatic  designs  are 
seen  by  the  aid  of  light  which  they  reflect  in  a diffused  way 
to  the  eye  of  the  observer  ; transparencies,  designs  in  stained 
or  painted  glass,  etc.,  are,  on  the  other  hand,  seen  by  light 
which  passes  entirely  through  their  substance  before  reach- 
ing the  eye.  Corresponding  to  this  we  find  that  by  far  the 
larger  proportion  of  natural  objects  act  upon  oim  visual 
organs  by  means  of  reflected  light,  while  a few  only  are 
seen  by  a mixture  of  reflected  and  transmitted  light.  It 
hence  follows  that  Nature  and  the  painter  actually  employ, 
in  the  end,  exactly  the  same  means  in  acting  on  the  eye  of 
the  beholder.  This  point,  seemingly  so  trite,  is  touched 
upon,  as  an  idea  seems  to  prevail  in  the  minds  of  many  per- 
sons that  Nature  paints  always  with  light,  while  the  artist 
is  limited  to  pigments  : in  point  of  fact,  both  paint  with 
light,  though,  as  we  shall  hereafter  see,  the  total  amount  at 
the  disposal  of  the  painter  is  quite  limited. 


THE  EEFLEOTION  AND  TRANSMISSION  OF  LIGHT.  I5 

In  concluding  this  matter  of  reflection,  we  may  perhaps 
be  allowed  to  add  that  the  term  reflection  is  quite  frequent- 
ly confused  with  shadow — the  reflected  image  of  trees  on 
the  edge  of  quiet  water  being  often  spoken  of  as  the  shadows 
of  trees  on  the  water.  The  two  cases  are  of  course  essen- 
tially different,  a genuine,  well-deflned  shadow  on  water 
scarcely  occurring  except  in  cases  of  turbidity. 

We  have  seen  that  all  bodies  reflect  some  of  the  light 
falling  on  them  ; it  is  equally  true  that  they  transmit  a 
certain  portion.  A plate  of  very  pure  glass,  or  a thin  layer 
of  pure  water,  will  transmit  all  the  light  falling  on  it,  ex- 
cept that  which  is  reflected  ; they  transmit  it  unaltered  in 
tint,  and  we  say  they  are  perfectly  transparent  and  colour- 
less substances.  Here  we  have  one  of  the  extremes  ; the 
other  may  be  found  in  some  of  the  metals,  such  as  gold  or 
silver  : it  is  only  when  they  are  reduced  to  very  thin  leaves 
that  they  transmit  any  light  at  all.  Gold  leaf  allows  a lit- 
tle light  to  pass  through  its  substance,  and  tinges  it  bluish- 
green.  Almost  all  other  bodies  may  be  ranged  between 
these  two  examples  ; none  can  be  considered  absolutely 
transparent,  none  perfectly  opaque.  And  this  is  true  not 
only  in  a strictly  philosophical  sense,  but  also  in  one  that 
has  an  especial  bearing  on  our  subject.  The  great  mass  of 
objects  with  which  we  come  in  daily  contact  allow  light  to 
penetrate  a little  way  into  their  substance,  and  then,  turn- 
ing it  back,  reflect  it  outward  in  all  directions.  In  this 
sense  all  bodies  have  a certain  amount  of  transparency. 
The  light  which  thus,  as  it  were,  just  dips  into  their  sub- 
stance, has  by  this  operation  a change  impressed  on  it ; it 
usually  comes  out  more  or  less  coloured.  It  hence  follows 
that,  in  most  cases,  two  masses  of  light  reach  the  eye  : one, 
which  has  been  superficially  reflected  with  unchanged  colour; 
and  another,  which,  being  reflected  only  after  penetration, 
is  modified  in  tint.  Many  beautiful  effects  of  translucency 
are  due  to  these  and  strictly  analogous  causes  ; the  play 


16 


MODERN  CHROMATICS. 


of  colour  on  the  surfaces  of  waves  is  made  up  largely  of 
these  two  elements  ; and  in  a more  subdued  way  we  find 
them  also  producing  the  less  marked  translucency  of  foliage 
or  of  flesh. 

One  of  the  resources  just  mentioned  the  painter  never 
employs : the  light  which  is  more  or  less  regularly  reflected 
from  the  outermost  surface,  he  endeavours  to  prevent  from 
reaching  the  eye  of  the  beholder,  except  in  minute  quanti- 
ty, his  reliance  being  always  on  the  light  which  is  reflected 
in  an  irregular  and  diffused  way,  and  which  has  for  the 
most  part  penetrated,  first,  some  little  distance  into  his 
pigments. 

The  glass-stainer  and  glass-painter  make  use  of  the 
principle  of  the  direct  transmission  of  light  for  the  display 
of  their  designs.  Now,  as  painted  or  stained  glass  trans- 
mits enormously  more  light  than  pigments  reflect  in  a prop- 
erly lighted  room,  it  follows  that  the  worker  on  glass  has  at 
his  disposal  a much  more  extensive  scale  of  light  and  shade 
than  the  painter  in  oils  or  water-colours.  Owing  to  this 
fact  it  is  possible  to  produce  on  glass,  paintings  which,  in 
range  of  illumination,  almost  rival  Nature.  The  intensity 
and  purity  of  the  tints  which  can  thus  be  produced  by 
direct  transmission  are  far  in  advance  of  what  can  be  ob- 
tained by  the  method  of  reflection,  and  enable  the  designer 
on  glass  successfully  to  employ  combinations  of  colour 
which,  robbed  of  their  brightness  and  intensity  by  being 
executed  in  oils  or  fresco,  would  no  longer  be  tolerable. 


CHAPTER  II. 

PRODUCTION  OF  COLOUR  BY  DISPERSION. 

In  the  previous  chapter  we  have  seen  that  the  sensation 
of  sight  is  produced  by  the  action  of  very  minute  waves  on 
the  nervous  substance  of  the  retina  ; that  is  to  say,  by  the 
aid  of  purely  mechanical  movements  of  a definite  character. 
When  these  waves  have  a length  of  about  inch, 

they  produce  the  sensation  which  we  call  red — we  see  red 
light ; if  they  are  shortened  to  of  an  inch,  their  ac- 

tion on  us  changes,  they  call  up  in  us  a different  sensation 
— we  say  the  light  is  coloured  orange  ; and  as  the  lengths 
of  the  waves  are  continually  shortened,  the  sensation  passes 
into  yellow,  green,  blue,  and  violet.  From  this  it  is  evident 
that  colour  is  something  which  has  no  existence  outside  and 
apart  from  ourselves  ; outside  of  ourselves  there  are  merely 
mechanical  movements,  and  we  can  easily  imagine  beings 
so  constructed  that  the  waves  of  light  would  never  produce 
in  them  the  sensation  of  colour  at  all,  but  that  of  heat. 

The  colour-sensations  just  mentioned  are  not  the  only 
ones  capable  of  being  produced  by  the  gradual  diminution 
of  the  wave-length  : between  the  red  and  orange  we  find 
every  variety  of  orange-red  and  red-orange  hue  ; the  or- 
ange, again,  changes  by  a vast  number  of  insensible  steps 
into  yellow,  and  so  of  all  the  other  tints.  Types  of  all 
colours  possible,  except  the  purples,  could  be  produced  by 
this  method.  The  colours  generated  in  this  way  would  not 
only  pass  by  the  gentlest  gradations  into  each  other,  form- 
ing a long  series  of  blending  hues,  but  they  would  also  be 


18 


MODERN  CHROMATICS. 


perfectly  pure,  and,  if  the  light  was  bright,  very  intense. 
The  advantage  of  providing,  in  the  beginning  of  our  colour 
studies,  a set  of  tints  possessing  these  precious  qualities,  is 
evident  without  much  argument. 

Kow,  white  light  consists  of  a mixture  of  waves  pos- 
sessing every  desirable  degree  of  length,  and  it  is  only  ne- 
cessary to  select  some  instrument  which  is  able,  to  sort  out 
for  us  the  different  kinds  of  light,  and  neatly  arrange  them 
side  by  side  in  an  orderly  series.  Fortunately  for  us,  we 
find  in  the  glass  prism  a simple  and  inexpensive  apparatus 
which  is  able  to  effect  the  desired  analysis.  We  may,  if  we 
are  willing  to  take  a little  trouble,  arrange  matters  so  as  to 


Fig.  1.— Prismatic  Spectrum. 


repeat  the  famous  experiment  made  by  Newton  many  years 
ago  : viz.,  admit  a small  beam  of  sunlight  into  a darkened 
room,  and  allow  it  to  fall  on  the  prism,  as  indicated  in  Fig. 
1.  We  shall  notice,  by  observing  the  illuminated  path  of 
the  sunbeam,  that  the  prism  bends  it*  considerably  out  of 
its  course ; and,  on  tracing  up  this  deflected  portion,  we 
shall  find  it  no  longer  white,  but  changed  into  a long  streak 
of  pure  and  beautiful  colours,  which  blend  into  each  other 
by  gentle  gradations.  If  this  streak  of  coloured  light  be 
received  on  a white  wall,  or,  better,  on  a large  sheet  of 
white  cardboard,  the  following  changes  in  the  colours  can 


PRODUCTION  OF  COLOUR  BY  DISPERSION. 


19 


be  noticed : It  commences  at  one  end  with  a dark- crimson 
hue,  which  gradually  brightens  as  we  advance  along  its 
length,  changing  at  the  same  time  into  scarlet ; this  runs 
into  orange,  the  orange  becomes  more  yellowish,  and  con- 
trives to  convert  itself  into  a yellowish-green  without  pass- 
ing noticeably  into  yellow,  so  that  at  first  sight  yellow  does 
not  seem  to  be  present.  The  orange-yellow  and  greenish- 
yellow  spaces  are  brighter  than  any  of  the  others,  but  the 
rise  in  luminosity  is  so  gradual  that  the  difference  is  not 
striking,  unless  we  compare  these  two  colours  with  those  at 
a considerable  distance  from  them.  As  we  pass  on,  the  ten- 
dency to  green  becomes  more  decided,  until  finally  a full 
green  hue  is  reached.  This  colour  is  still  pretty  bright, 


Fig.  2. — Mode  of  isolating  a Single  Colour  of  the  Prismatic  Spectrum. 


and  not  inferior  to  the  red  in  intensity  ; by  degrees  it 
changes  into  a greenish-blue,  which  will  not  at  first  attract 
the  attention  ; next  follows  a full  blue,  not  nearly  so  bright 
as  the  green,  nor  so  striking  ; this  blue  changes  slowly  into 
a violet  of  but  little  brightness,  which  completes  the  series. 
If  we  wish  to  isolate  and  examine  these  tints  separately, 
we  can  again  follow  the  example  of  IsTewton,  by  making  a 
small,  narrow  aperture  in  our  cardboard,  and  use  it  then  as 
a screen  to  intercept  all  except  the  desired  tint,  as  is  indi- 
cated in  Fig.  2.  In  this  manner  we  can  examine  separate 


20 


MODERN  CHROMATICS. 


portions  of  oiir  spectrum  more  independently,  and  escape 
from  the  overpowering  influence  of  some  of  the  more  in- 
tense tints.  Under  these  circumstances  the  greenish-blue 
becomes  quite  marked,  and  the  blue  is  able  to  assert  itself 
to  a greater  degree ; but  the  yellow  will  not  be  greatly 
helped,  for  in  fact  it  is  confined  to  a very  narrow  region, 
and  it  is  only  by  greatly  magnifying  the  spectrum  that  we 
can  obtain  a satisfactory  demonstration  of  its  existence. 

These  experiments,  though  very  beautiful,  are  quite 
rough  ; every  two  minutes  the  beam  of  sunlight  strays 
away  from  the  prism  and  needs  again  to  be  directed  toward 
it ; and  besides  that,  the  colours  blend  into  each  other  in 
such  a subtile,  puzzling  way,  that,  without  a scale  or  land- 
mark of  some  kind  to  separate  them,  it  seems  hopeless  to 
undertake  any  exact  experiments.  In  tliis  ditliculty  it  is  to 
the  spectroscope  that  we  must  turn  for  aid  ; it  was  certainly 
not  originally  contrived  for  such  j)ur])oses  as  these,  but 
nevertheless  is  just  wliat  we  need.  It  is  not  necessary  to 
stop  to  describe  the  instrimient,  as  this  lias  been  done  by 
Professor  Lommel  in  another  volume  of  this  series  ; it  is 
enough  for  us  that  it  is  a convenient  instrument  for  sorting 
out  the  different  kinds  of  light  which  fall  on  it,  according 
to  their  wave-length,  and  that  it  performs  this  work  far 
more  accurately  than  a jirism  used  according  to  Newton’s 
plan.  Just  at  this  point  we  can  take  advantage  of  a sin- 
gular discovery  made  by  Fraunhofer,  and  independently 
to  some  extent  by  Dr.  AVollaston,  early  in  the  present  cen- 
tury. These  physicists  found  that  when  the  coloured  band 
of  light  just  described  is  produced  by  a spectroscope,  or 
by  apparatus  equivalent  to  one,  the  band  is  really  not  con- 
tinuous, but  is  cut  up  crosswise  into  a great  many  small 
spaces.  The  dividing  lines  are  called  the  fixed  lines  of  the 
solar  spectrum.  Almost  their  sole  interest  for  us  is  in  the 
fact  that  they  serve  as  admirable  landmarks  to  guide  us 
through  the  vague  tracts  of  ill-defined  colour.  Fig.  3 shows 
the  positions  of  some  of  the  more  important  fixed  lines  of 


PRODUCTION  OF  COLOUR  BY  DISPERSION. 


21 


* It  will  be  noticed  that  the  term  indigo^ 
originally  introduced  by  Newton,  has  been 
entirely  rejected  in  this  work,  and  ultrama- 
rine substituted  for  it.  Bezold  suggested 
this  change  some  time  ago,  basing  his  ob- 
jection to  indigo  on  its  dinginess ; the  au- 
thor, however,  finds  a much  more  fatal  ob- 
jection in  the  fact  that  indigo  in  solution, 
and  as  a pigment,  is  a somewhat  greenish- 
blue,  being  really  identical  with  Prussian- 
blue  in  colour,  only  far  blacker.  In  the 
dry  state  this  tendency  to  greenness  is  neu- 
tralized by  the  reddish  tinge  which  the  sub- 
stance sometimes  assumes : it  was  probably 
used  by  Newton  in  the  dry  state.  A mix- 
ture of  six  parts  of  artificial  ultramarine- 
blue,  two  parts  white,  and  ninety-two  parts 
black,  when  mingled  according  to  the  meth- 
od of  Maxwell’s  disks,  furnishes  a colour 
quite  like  that  of  commercial  indigo  in  the 
dry  state. 


sEed. 


the  spectrum.  The  figure  is  based  on  measurements  made 
by  the  author  on  a flint-glass  prism, 
with  aid  of  a large  spectroscope, 
or  rather  spectrometer,  admirably 
constructed  by  Wm.  Grunow,  of 
New  York.  At  the  same  time  a 
series  of  observations  was  made 
on  the  extent  of  the  coloured 
spaces  in  the  spectrum  ; these  are 
indicated  in  the  figure,  and  ac- 
curately given  in  one  of  the  ta- 
bles that  follow.*  Let  us  sup- 
pose that  the  spectrum  from  A to 
H includes  1,000  parts ; then  the 
following  table  indicates  the  po- 
sitions of  the  fixed  lines  : 


Eed-orange. 

Orange. 

Orange-yellow. 

Yellow. 


Green-yellow 
and 
Yellow-green. 


Green  and 
Blue-green. 


> 


Cyan-blue. 


V Blue  and 
? Blue-violet. 


►Violet 


Fig.  3.— Fixed  Lines  and  Coloured 
Spaces  of  Prismatic  Spectrum. 


22 


MODERN  CHROMATICS. 


Fixed  Lines  of  the  Prismatic  Spectrum. 


0 

E 

363-11 

40-05 

b 

389-85 

74-02 

F 

493-22 

112-71 

G 

753-58 

220-31 

II 

1000-00 

The  next  table  gives  tbe  positions  of  the  coloured  spaces 
in  this  spectrum,  according  to  the  observations  of  the  author  : 

Coloured  Spaces  in  the  Prismatic  Spectrum. 


Red  begins  at 0 

Red  ends,  orange-red  begins  at 149 

Orange-red  ends,  orange  begins  at 194 

Orange  ends,  orange-yellow  begins  at 210 

Orange-yellow  ends,  yellow  begins  at 230 

Yellow  ends,  grcenisli-yellow  begins  at 240 

Yellow-green  ends,  green  begins  at 344 

Blue-green  ends,  cyan-blue  begins  at ' 447 

Cyan-blue  ends,  blue  begins  at 495 

Violet-blue  ends,  violet  begins  at 806 

Violet  ends  at 1,000 


The  space  out  beyond  0 is  occupied  by  a very  dark  red, 
which  has  a brown  or  chocolate  colour,  and  outside  of  the 
violet  beyond  1,000  is  a faint  greyish  colour,  which  has  been 
called  lavender. 

The  third  table  shows  the  spaces  occupied  in  the  pris- 
matic  spectrum  by  the  several  colours  : 


Red 149 

Orange-red 45 

Orange 16 

Orange-yellow 20 

Yellow 10 

Greenish-yellow  and  yellowish-green 104 

Green  and  blue-green 103 

Cyan-blue 48 

Blue  and  blue-violet 311 

Violet 194 


,000 


PEODUCTION  OF  COLOUR  BY  DISPERSION, 


23 


In  making  tkese  observations,  matters  were  arranged  so 
that  only  a narrow  slice  of  the  spectrum  presented  itself  to 
the  observer  ; thus  its  hues  could  be  studied  in  an  isolated 
condition,  and  the  misleading  effects  of  contrast  avoided. 
The  figures  given  in  the  two  latter  tables  are  the  mean  of 
from  fifteen  to  twenty  observations.  The  hues  of  the  spec- 
tral colours  change  very  considerably  with  their  luminosity  ; 
hence  for  these  experiments  an  illumination  was  selected 
such  that  it  was  only  comfortably  bright  in  the  most  lumi- 
nous portions  of  the  spectrum,  and  this  arrangement  re- 
tained as  well  as  possible  afterward. 

The  colours  as  seen  in  the  spectroscope  really  succeed 
each  other  in  the  order  of  their  wave-lengths,  the  red  hav- 
ing the  greatest  wave-length,  the  violet  the  least.  But  the 
glass  prism  does  this  work  in  a way  which  is  open  to  criti- 
cism ; it  crowds  together  some  portions  of  the  series  of  tints 
more  than  is  demanded  by  their  difference  in  wave-lengths  ; 
other  portions  it  expands,  assigning  to  them  more  room 
than  they  have  a right  to  claim.  Thus  the  red,  orange, 
and  yellow  spaces  are  cramped  together,  while  the  blue  and 
violet  tracts  stretch  out  interminably.  Taking  all  this  into 
consideration,  it  may  be  worth  while  to  go  one  step  further, 
and,  without  abandoning  the  use  of  the  spectroscope,  re- 
place its  prism  by  a diffraction  grating,  or  plate  of  glass 
ruled  with  very  fine,  parallel,  equidistant  lines,  such  as  have 
been  made  by  the  celebrated  Il^obert,  and  lately  of  still 
superior  perfection  by  Rutherfurd.  In  Lommel’s  work, 
previously  referred  to,  the  mode  in  which  a plate  of  this 
kind  produces  colour  is  explained  ; at  present  it  is  enough  to 
know  that  the  general  appearance  of  the  spectacle  will  be 
unchanged  ; the  same  series  of  colours,  the  same  fixed  lines, 
will  again  be  recognized  ; but  in  this  new  spectrum  all  the 
tints  will  be  arranged  in  an  equable  manner  with  reference 
to  wave-length.  According  to  this  new  allotment  of  spaces, 
the  yellow  will  occupy  about  the  centre  of  the  spectrum, 


24 


MODERN  CHROMATICS. 


the  red  and  different  kinds  of  orange  taking  up  more  room 
than  formerly  ; the  dimensions  of  the  blue  and  violet  will 
be  greatly  reduced. 

Let  ns  suppose,  as  before,  that  the  spectrum  from  A to 
H includes  1,000  parts  ; then  the  following  table,  which  is 
calculated  from  the  observations  of  Angstrom,  will  indi- 
cate the  positions  of  the  principal  fixed  lines  : 

Fixed  Lines  in  the  Normal  Spectrum. 


0 

E 

038-92 

113Y4 

h 

664-79 

201-01 

F 

749-24 

285-05 

G 

902-07 

468-38 

II 

1000-00 

The  next  table  gives  the  positions  of  the  coloured  spaces 
in  the  normal  spectrum,  according  to  the  observations  of 
the  author  : 


Coloured  Spaces  in  the  Normal  Spectrum. 


Red  begins  at 0 

Pure  red  ends,  orange-red  begins  at 380 

Orange-red  ends,  orange  begins  at 434 

Orange  ends,  orange-yellow  begins  at 459 

Orange-yellow  ends,  yellow  begins  at 485 

Yellow  ends,  greenisb -yellow  begins  at 498 

Yellow-green  ends,  full  green  begins  at 595 

Full  green  ends,  blue-green  begins  at 682 

Blue-green  ends,  eyan-blue  begins  at 098 

Cyan-blue  ends,  blue  begins  at 149 

Blue  ends,  violet-blue  begins  at 823 

Blue-violet  ends,  pure  violet  begins  at 040 


The  following  table  exhibits  the  spaces  occupied  by  the 
several  colours  in  the  normal  spectrum  : 


Pure  red 

330 

Oranf>-e-red 

104 

25 

26 

PRODUCTION  OF  COLOUR  BY  DISPERSION. 


25 


Yellow 13 

Greenish-yellow  and  yellow-green . 97 

Full  green 87 

Blue-green 16 

Cyan-blue 51 

Blue 74 

Violet-blue  and  blue-violet 117 

Pure  violet. GO 

1,00,0 


n 


Orangre-red, 


Orange. 

Orange-yellow. 

Yellow. 

Greenish-yellow 


T 


Yellowish -green. 


Green. 


Fig.  4 shows  the  normal 
spectrum  with  fixed  lines  and 
coloured  spaces,  corresponding 
to  the  tables  just  given. 

If  these  tables  are  compared 
with  those  obtained  by  the  aid 
of  a prism  of  glass,  it  will  be 
seen  that  the  fixed  lines  and 
coloured  spaces  are  arranged 
somewhat  differently ; the 
main  cause  of  this  difference 
has  already  been  pointed  out. 

When,  however,  we  compare 
the  spacing  of  the  colours  in 
the  two  spectra,  it  is  also  to  be 
remembered  that  it  is  affected 
by  another  circumstance,  viz., 
the  distribution  of  the  lumi- 
nosity in  the  two  spectra  does 
not  agree,  and  this  influences, 
as  will  be  shown  in  Chapter 
XII.,  the  appearance  of  the 
colours  themselves  ; very  lu- 
minous red,  for  example,  as- 
suming an  orange  hue,  very 
dark  blue  tending  to  appear 

violet,  etc.  The  normal  spectrum  employed  by  the  author 


Red. 


^ Blue-gveen. 
V Cyan-blue. 

Blue. 


Violet-blue. 


"iolet. 


Fig.  4. — Fixed  Lines  and  Coloured 
Spaces  of  Normal  Spectrum. 


26 


MODERN  CIIUOMATIOS. 


was  obtained  by  using  a superb  plate  for  whieb  he  was 
indebted  to  Mr.  Rutherfurd.  The  plate  contained  nearly 
19,000  lines  to  the  English  inch,  and  was  silvered  on  the 
back,  so  that  the  colours  were  as  bright  as  those  from  a 
glass  prism.  The  spectrum  selected  for  use  rvas  nearly  si.v 
times  as  long  as  that  furnished  by  the  glass  prism-a  cir- 
cumstance, of  course,  that  favoured  accurate  observation. 

The  tables  that  have  just  been  given  enable  us  very 
easily  to  calculate  the  lengths  of  the  w.aves  of  light,  cor- 
responding to  the  centres  of  the  coloured  spaces  in  the  nor- 
mal spectnuu.  It  is  only  necessary  to  ascertain  the  number 
corresponding,  for  example,  to  the  centre  of  the  red  space, 
then  to  multiply  it  by  IhC,-):!,  and  to  subtract  the  lu'oduct 
from  7,606  : the  result  will  be  the  wave-length  correspond- 
ing to  that  part  of  the  normal  spectrum,  expressed  m ten- 
millionths  of  a millimetre.  The  tollowing  table  contains 
the  wave-lengths  corresponding  to  the  centres  of  the  col- 
oured  spaces  : 


Wave-length  in  tc.-oTioToCTu 

Cent'-e  of  retl 

“ orange- veil 

“ orange 

“ orange-yellow 

“ yellow 

“ lull  green 

blne-grren 

“ cyan-blue 

“ blue 

“ violet-blue 

“ pure  violet 

Tlie  results  here  given  differ  somewhat  from  those  obtained 
by  Listing  in  1867  ; the  differences  are  partly  due  to  the 
terms  employed  ; the  author,  for  example,  dividing  up  into 
orange-red,  orange,  and  orange-yellow,  a space  which  is 
called  by  Listing  simply  orange.  According  to  the  author 


7,ooa 

f.,208 

r.,072 

r),8Ta 

.^),80S 

r>,27i 

5,082 

4,732 

4,383 

4,059 


PRODUCTION  OF  COLOUR  BY  DISPERSION. 


27 


cyan-blue  falls  on  the  red  side  of  the  line  F ; it  is  placed 
by  Listing,  however,  on  the  violet  side  of  this  line.  Other 
less  important  differences  might  be  mentioned  ; but,  as  a dis- 
cussion of  them  would  be  out  of  place  in  a work  like  the 
present,  the  curious  reader  is  referred  for  further  informa- 
tion to  Listing’s  paper.* 

A little  study  of  the  normal  spectrum.  Fig.  4,  will  enable 
us  to  answer  some  interesting  questions.  We  have  already 
seen  that  change  in  colour  is  always  accompanied  by  change 
in  the  length  of  the  waves  of  light  producing  it  ; hence  if  we 
begin  at  one  end  of  our  normal  spectrum  where  the  colour 
is  red,  and  the  length  of  the  wa^^es  equal  to  7,603  ten- 
millionths  of  a millimetre,  as  we  diminish  this  length,  we 
expect  to  see  a corresponding  change  in  the  colour  of  the 
light  : small  changes  we  anticipate  will  produce  small  effects 
on  the  colour,  large  changes  greater  effects. 

Now,  the  question  arises  whether  equal  changes  of  wave- 
length actually  are  accompanied  by  equal  alterations  of  hue 
in  all  parts  of  the  spectrum.  To  take  an  example  : in  pass- 
ing from  the  orange-yellow,  through  the  pure  yellow  and 
greenish-yellow  well  into  the  yellow-green  region,  we  find 
it  necessary  to  shorten  our  wave-length  about  400  of  our 
units  ; now  will  an  equal  curtailment  in  other  regions  of  the 
spectrum  carry  us  through  as  many  changes  of  hue  ? The 
answer  to  this  is  not  exactly  what  we  might  expect.  In  a 
great  part  of  the  red  region  a change  of  this  kind  produces 
only  slight  effects,  the  red  inclining  a little  more  or  less  to 
orange,  and  the  same  is  true  of  the  blue  and  violet  spaces, 
the  hue  leaning  only  a little  toward  the  blue  or  violet  side, 
as  the  case  may  be.  Hence  it  seems  that  the  eye  is  far 
more  sensitive  to  changes  of  wave-length  in  the  middle 
regions  of  the  spectrum  than  at  either  extremity.  This 
circumstance,  to  say  the  least,  is  curious  ; but,  what  is  more 
to  our  purpose,  it  is  a powerful  argument  against  any  theory 


* Poggendorff’s  “ Annalen/’  cxxxi.,  p.  564. 


28 


MODERN  CHROMATICS. 


of  colour  wliich  is  founded  on  supposed  analogies  with 
music.  But  more  of  tliis  hereafter. 

In  the  prismatic  spectrum  and  in  our  normal  spectrum 
we  found  no  representative  of  purple,  or  purplish  tints. 
This  sensation  can  not  he  produced  by  one  set  of  waves 
alone,  whatever  their  length  may  be  ; it  needs  the  joint 
action  of  the  red  and  violet  waves,  or  the  red  and  blue.  All 
other  possible  tints  and  hues  find  their  type  in  some  portion 
of  the  spectrum,  and,  as  will  be  shown  in  the  next  chapter, 
this  applies  just  as  well  to  the  wliole  range  of  browns  and 
greys,  as  to  colours  like  vermilion  and  ultramarine. 

We  have  seen  that  the  mixture  of  long  and  short  waves 
which  compose  white  light  can  be  analyzed  by  a prism  into 
its  original  constituents  : the  long  Avaves  ju-oduce  on  us  the 


Fig.  5.— Kecomposition  of  White  Light. 


sensation  that  we  call ' red,  and,  as  we  allow  shorter  and 
shorter  Avaves  to  act  on  the  eye,  Ave  experience  the  sensa- 
tions knoAvn  as  orange,  yclloAv,  green,  blue,  and  violet. 

hen,  on  the  other  hand,  aa'c  combine  or  mix  together  these 
different  kinds  of  light,  we  reproduce  Avhite  light.  There 
are  a great  many  different  Avays  of  effecting  this  recom- 


PRODUCTION  OF  COLOUR  BY  DISPERSION. 


29 


position  ; one  of  the  most  beautiful  was  contrived  several 
years  ago  by  Professor  Eli  Blake.  The  spectrum  is  re- 
ceived on  a strip  of  ordinary  looking-glass,  which  is  gently 
bent  by  the  hands  of  the  experimenter  till  it  becomes  some- 
what curved  ; it  then  acts  like  a concave  mirror,  and  can  be 
made  to  concentrate  all  the  coloured  rays  on  a distant  sheet 
of  paper,  as  shown  in  Fig.  5.  The  spot  where  all  the  col- 
oured rays  are  united  or  mixed  appears  pure  white. 


CHAPTER  HI. 


THE  CONSTANTS  OF  COLOUR. 

The  tints  produced  by  Aatiire  and  art  are  so  manifold, 
often  so  vague  and  indetinite,  so  affected  by  their  environ- 
ment, or  by  the  illumination  under  vliieh  they  are  seen, 
that  at  first  it  might  well  ajjpear  as  though  nothing  about 
them  were  constant  ; as  though  they  had  no  fixed  proper- 
ties which  could  be  used  in  reducing  them  to  order,  and  in 
arranging  in  a simple  but  vast  series  the  immense  multitude 
of  which  they  consist. 

Let  us  examine  the  matter  more  closely.  We  have  seen 
that  when  a single  set  of  waves  acts  on  the  eye  a colour- 
sensation  is  ])roduced,  which  is  j)erfeetly  well  detined,  and 
which  can  be  indicated  with  ju'ecisiun  by  referring  it  to 
some  portion  of  the  spectrum.  We  have  also  found  that 
when  waves  of  light,  having  all  possible  lengths,  act  on  the 
eye  simultaneously,  the  sensation  of  white  is  j»roduced. 
Let  us  suppose  that  by  the  iirst  method  a delinite  colour- 
sensation  is  generated,  and  afterward,  by  the  second  meth- 
od, the  sensation  of  white  is  abided  to  it  : white  light  is 
added  to  or  mixed  with  coloured  light.  This  mixture  may 
be  accoin})lished  by  throwing  the  solar  spectrum  on  a large 
sheet  of  white  paper,  and  then  casting  on  the  same  sheet  of 
paper  the  white  light  which  is  reflected  from  a silvered 
mirror,  or  from  an  unsilvered  jdate  of  glass.  Fig.  6 shows 
the  arrangement.  By  moving  the  mirror  W,  Fig.  6,  the 
white  band  of  light  may  be  made  to  travel  slowly  over  the 
whole  spectrum,  and  thus  furnish  a scries  of  mixtures  of 


THE  CONSTANTS  OF  COLOUR. 


31 


white  light  with  the  various  prismatic  hues.  The  general 
effect  of  this  proceeding  will  be  to  diminish  the  action  of 
the  coloured  light ; the  mixture  will  indeed  present  to  the 
eye  more  light,  but  it  will  be  paler  ; the  colour-element  will 
begin  to  be  pushed  into  the  background.  Conversely,  if 
we  now  should  subject  our  mixture  of  white  and  coloured 
light  to  analysis  by  a second  prism,  we  should  infallibly 
detect  the  presence  of  the  white  as  well  as  of  the  coloured 
light  ; or,  if  no  white  light  were  present,  that  would  also 


Fig.  6. — Mode  of  mixing  White  Light  with  the  Colours  of  the  Spectrum. 


be  equally  apparent.  Taking  all  this  into  consideration,  it 
is  evident  that,  when  a particular  colour  is  presented  to  us, 
we  can  affirm  that  it  is  perfectly  pure  ; viz.,  entirely  free 
from  white  light,  or  that  it  contains  mingled  with  it  a 
larger  or  smaller  proportion  of  this  foreign  element.  This 
furnishes  us  with  our  first  clue  toward  a classification  of 
colours  : our  pure  standard  colours  are  to  be  those  found  in 
the  spectrum  ; the  coloured  light  coming  from  the  surfaces 
of  natural  objects,  or  from  painted  surfaces,  we  must  com- 
pare with  the  hues  of  the  spectrum.  If  this  is  done,  in  al- 
most every  case  the  presence  of  more  or  less  white  light 
will  be  detected  ; in  the  great  majority  of  instances  its 


32 


MODERN  CHROMATICS. 


preponderance  over  the  coloured  light  will  he  found  quite 
marked.  To  illustrate  hy  an  example  : if  white  paper  be 
painted  with  vermilion,  and  compared  with  the  solar  s])ec- 
trum,  it  will  be  noticed  that  it  corresponds  in  general  hue 
with  a certain  portion  of  the  red  space  ; but  the  two  colours 
never  match  perfectly,  that  from  the  paper  always  appear- 
ing too  pale.  If,  now,  white  light  be  added  to  the  pure 
spectral  tint,  by  reflecting  a small  amount  of  it  from  the 
mirror  (Fig.  G),  it  will  become  possible  to  match  the  two 
colours  ; and,  if  we  know  how  much  white  light  has  been 
added,  we  can  afterward  say  that  the  light  reflected  from 
the  vermilion  consists,  for  example,  of  eighty  per  cent,  of 
red  light  from  such  a region  of  tlie  spectrum,  mixed  with 
twenty  per  cent,  of  white  light.  If  we  make  the  experi- 
ment with  a surface  painted  with  ‘‘  emerald  green,”  we 
shall  obtain  about  the  same  result,  while  we  shall  lind  that 
artificial  ultramarine-blue  reflects  about  twenty-live  per 
cent,  of  white  light.  In  all  of  these  cases  the  total  amount 
of  light  reflected  by  the  coloured  j)aper  is  of  course  taken 
as  100,  and  the  results  here  given  arc  to  be  regarded  only 
as  approximations.  In  every  case  some  white  light  is  sure 
to  be  present  ; its  clYcct  is  to  soften  the  colour  and  reduce  its 
action  on  the  eye  ; when  the  pro])ortion  of  white  is  very 
large,  only  a faint  reminiscence  of  the  original  hue  remains  : 
we  say  the  tint  is  greenish-grey,  bluish-grey,  or  rcddisli- 
grey.  If  one  part  of  red  light  is  mixed  with  sixteen  parts 
of  white  light,  the  mixture  a]ipears  of  a pale  pinkish  line. 
The  specific  effects  produced  by  the  mixture  of  white  with 
coloured  light  will  be  considered  in  Chapter  XII.  ; it  is 
enough  for  us  at  present  to  have  obtained  an  i<lea  of  one  of 
the  constants  of  colour,  viz.,  its  purity.  The  same  word,  it 
may  be  observed,  is  often  used  by  artists  in  an  entirely  dif- 
ferent sense  : they  will  remark  of  a painting  that  it  is  no- 
ticeable for  the  purity  of  its  colour,  meaning  only  that  the 
tints  in  it  have  no  tendency  to  look  dull  or  dirty,  but  not  at 
all  implying  the  absence  of  white  or  grey  light. 


THE  CONSTANTS  OF  COLOUR. 


Next  let  us  suppose  that  in  our  study  of  these  matters 
we  have  presented  to  us  for  examination  two  coloured  sur- 
faces, which  we  find  reflect  in  both  cases  eight  tenths  red 
light  and  two  tenths  white  light.  In  spite  of  this,  the  tints 
may  not  match,  one  of  them  being  much  brighter  than  the 
other  ; containing,  perhaps,  twice  as  much  red  light  and 
twice  as  much  white  light  ; having,  in  other  words,  twice 
as  great  brightness  or  luminosity.  The  only  mode  of  caus- 
ing the  tints  to  match  will  be  to  expose  the  darker-coloured 
surface  to  a stronger  light,  or  the  brighter  surface  to  one 
that  is  feebler.  It  is  evident,  then,  that  brightness  or  lumi- 
nosity is  one  of  the  properties  by  which  we  can  define  col- 
our ; it  is  our  second  colour-constant.  This  word  luminos- 
ity is  also  often  used  by  artists  in  an  entirely  different  sense, 
they  calling  colour  in  a painting  luminous  simply  because  it 
recalls  to  the  mind  the  impression  of  light,  not  because  it 
actually  reflects  much  light  to  the  eye.  The  term  “ bright 
colour  ” is  sometimes  used  in  a somewhat  analogous  sense 
by  them,  but  the  ideas  are  so  totally  different  that  there  is 
little  risk  of  confusion. 

The  determination  of  the  second  constant  is  practicable 
in  some  cases  ; it  presents  itself  always  in  the  shape  of  a 
dilficult  photometric  problem.  The  relative  brightness  of 
the  colours  of  the  solar  spectrum  is  one  of  the  most  inter- 
esting of  these  problems,  as  its  solution  would  serve  to  give 
some  idea  of  the  relative  brightness  of  the  colours  which, 
taken  together,  constitute  white  light.  Quite  recently *a 
set  of  measurements  was  made  in  different  regions  of  the 
spectrum  by  Yierordt,  who  referred  the  points  measured  to 
the  fixed  lines,  as  is  usual  in  such  studies."*^  Reducing  his 
designations  of  the  different  regions  of  the  spectrum  to 
those  of  our  spectral  chart,  which  includes  1,000  parts  from 
A to  H (see  previous  chapter),  and  supplying  the  colours 
from  the  observations  of  the  present  writer,  we  obtain  the 
following  table  : 

* C.  Vierordt,  Poggcndorff’s  “ Annalcn,”  Band  cxxxvii.,  S.  200. 


34 


MODERN  CHROMATICS. 


Table  showing  the  Luminosity  of  Different  Regions  of  the  Prismatic 

Spectrum. 


- 

Position. 

Luminosity,  j 

Coloui-. 

From 

40-5 

to 

57 

80  ^ 

Dark  red. 

(C 

104-5 

a 

112-71 

403 

Pure  red. 

U 

112-71 

u 

138-5 

1,100  ; 

Red. 

U 

158-5 

1G8-5 

2,773 

Orange-red. 

U 

180 

u 

220-31 

G,085 

Orange  and  orange-yellow. 

220-31 

u 

231-5 

7,801 

Orange-yellow. 

u 

231-5 

ii 

3G3-11 

3,033 

Greenish-yellow,  ycllow-green,  and  green. 

u 

380-85 

u 

403-22 

i 1,100 

Dluc-green  and  cyan-blue. 

U 

403-22 

u 

558-5 

403  1 

Dine. 

u 

G23-5 

iC 

G80-5 

00-G 

U 1 1 rainai  i nc  (art i tidal ). 

U 

753-58 

(( 

S25-5 

35-0  j 

Dine- violet. 

u 

80G-5 

u 

05G 

13-1  : 

Violet. 

The  author  liiids  that  witli  the  aiil  ot‘  rotating  disks  the 
second  constant  can  often  l)e  determined.*  Let  us  suppose 
that  we  wish  to  deterniine  the  luminosity  of  paper  painted 
with  vernidioii  ; a circular  disk,  about  six  inches  in  diame- 
ter, is  cut  from  the  paper  and  placed  on  a rotation  ajiparatus, 
as  indicated  in  Fig.  T.  On  the  same  axis  is  fastened  a double 
disk  of  black  and  of  white  paper,  so  arranged  that  the  pro- 
portions of  the  black  and  white  can  be  varied  at  wilLf  AVhen 
the  whole  is  set  in  rapid  rotation,  the  colour  of  the  vermilion 
paper  will  of  course  not  be  altered,  but  the  black  and  white 
will  blend  into  a grey.  This  grey  can  be  altered  in  its 
brio-htness,  till  it  seems  about  as  luminous  as  the  red  (Fig. 
8).  If  we  find,  for  example,  that  with  the  disk  three  quar- 
ters black  and  one  quarter  white  an  equality  appears  to  be 
established,  we  conclude  that  the  luminosity  of  our  red  sur- 
face is  twenty-five  per  cent,  of  that  of  the  white  paper.  This 


* “American  Journal  of  Science  and  Arts,”  Februarv,  1878. 
f See  Maxwell’s  “ Disks,”  chapter  x. 


THE  CONSTANTS  OF  COLOUR. 


35 


is  of  course  based  on  the  assumption  that  the  black  paper 
reflects  no  light ; it  actually  does  reflect  from  two  to  six 
per  cent.,  the  reflecting  power  of  white  paper  being  put  at 
100.  The  black  disk  used  by  the  author  reflected  5*2  per 


Fig,  T.— Coloured  Disk,  with  Small  Black-and- White 
Disk. 


Feg.  8.— Coloured  Disk, 
with  Small  Black-aud- 
White  Disk  iu  Eotation. 


cent,  of  white  light ; to  meet  this  a correction  was  intro- 
duced, and  a series  of  measurements  made,  some  of  the  more 
important  of  which  are  given  in  the  following  table  : 

Luminosity. 


White  paper 100 

Vermilion  (English)* 25’'7 

Pale  chrome-yellow  I 80‘3 

Pale  emerald-green  * 48‘6 

Cobalt-blue  f 85*4 

Ultramarine:); Y'G 


These  results  were  afterward  tested  by  the  use  of  a set 
of  disks,  the  colours  of  which  were  complementary  to  those 
mentioned  in  the  table,  and  these  additional  experiments 
and  calculations  showed  that  the  original  measurements 
ditfered  but  little  from  the  truth.  This  agreement  proved 
also  the  correctness  of  Grassmann’s  assumption,  that  the  total 

* In  thick  paste,  f Washed  on  as  a water-colour.  | Artificial,  as  a 
paste. 


36 


MODERN  CHROMATICS. 


intensity  of  the  mixture  of  masses  of  differently  coloured 
light  is  equal  to  the  sum  of  the  intensities  of  the  separate 
components. 

But  to  resume  our  search  for  colour-constants.*  We  may 
meet  with  two  portions  of  coloured  light  having  the  same 
degree  of  purity  and  the  same  api)arent  brightness,  which 
nevertheless  appear  to  the  eye  totally  different  : one  may 
excite  the  sensation  of  blue,  the  other  that  of  red  ; Ave  say 
the  hues  are  entirely  different.  The  hue  of  the  colour  is, 
then,  our  third  and  last  constant,  or,  as  the  physicist  would 
say,  the  degree  of  refraiigibility,  or  the  wave-length  of  the 
light.  In  the  preceding  clni))ter  it  has  been  shown  that  the 
spectrum  offers  all  possible  hues,  except  the  purj>les,  Avell 
arranged  in  an  orderly  series,  and  the  ])urples  themselves 
can  be  produced  with  some  trouble,  by  causing  the  blue  or 
violet  of  the  spectrum  to  mingle  in  certain  proportions  with 
the  red. 


Fig.  9.— Eye-piece  with  Dalton's  Scale. 


For  the  determination  of  the  hue,  an  ordinary  one-prism 
spectroscope  can  be  used  ; it  is  only  necessary  to  add  a little 
contriA'ance  Avhich  enables  the  obsei'A'er  to  isolate  at  will  any 


THE  CONSTANTS  OF  COLOUR. 


37 


small  portion  of  the  spectrum.  This  object  is  easily  at- 
tained by  introducing  into  the  eye-piece  of  the  instrument 


Fig.  10.— Eutherfurd’s  Automatic  Spectroscope. 

a diaphragm  perforated  by  a very  narrow  slit  (see  Fig.  9). 
The  observer  then  sees  in  the  upper  part  of  the  field  of  view 
the  selected  spectral  colour  ; in  the  lower  part  of  the  field 
the  scale  is  visible,  and  with  its  aid  the  precise  position  of 
the  prismatic  hue  can  be  determined.  Instead  of  using  a 


38 


MODERX  CHROMATICS. 


scale  divided  into  equal  parts,  it  is  often  advantageous  to 
employ  the  plan  suggested  by  Dr.  J.  0.  Dalton,  and  used 
by  him  for  determining  the  position  of  certain  absoiq:)tion 
bands.  Dr.  Dalton  employs  as  a scale  a minute  photograph 
which  shows  the  positions  of  the  fixed  lines,  and  divides  up 
the  spaces  between  them  conveniently.  Fig.  9 exhibits  the 
appearance  of  the  field  of  view  and  the  scale.  For  more 
accurate  work  Rutherfurd’s  automatic  sixq:)rism  spectro- 
scope can  be  employed  (see  Fig.  10*).  A difi’raction  grat- 
ing can  also  be  used — in  those  cases  where  the  student  of 
colour  is  so  fortunate  as  to  possess  one.f  With  a very  per- 
fect grating  of  this  kind,  for  which  the  author  was  indebted 
to  Mr.  Rutherfurd,  the  third  constant  was  determined  for  a 
number  of  coloured  disks.  The  following  table  gives  their 
positions  in  a normal  spectrum  having  from  A to  II  1,000 
parts  ; the  corresponding  wave-lengths  are  also  given  : 


N.irae  of  the  Colour. 

! 

Tosition  in  tho 
Normal  Spectrum. 

Wave-lcDfTth 

TuuuOuGu  Dim. 

Vermilion  (English) 

3S7 

0,200 

Red  lead 

P22  1 

6,001 

Pale  chromc-vcllow 

-ISS  : 

5,820 

Emerald-green 

018 

5,231 

Prussian-blue 

j 

•1,890 

Cobalt-blue 

: 770 

•1,790 

Ultramarine  (genuine) 

785  : 

4,735 

Ultramarine  (artifieial) 

857  ! 

4,472 

Same,  washed  with  lIolTmaun’s 

violet  j 

B.  P> 

010 

4,257 

We  have  seen  that  the  first  colour-constant  has  reference 
to  the  purity  of  the  colour,  or  indicates  the  relative  amount 
of  white  light  mixed  with  it.  This  constant  is  in  all  cases 

* Fig.  10  is  a facsimile  of  Rutherfurd’s  drawing  of  his  six-prism  spec- 
troscope (“American  Journal  of  Science  and  Arts,”  1865). 
f See  Chapter  iv.  for  an  account  of  the  grating. 


THE  CONSTANTS  OF  COLOUR. 


39 


difficult  to  determine  ; probably  something  might  be  effected 
by  carrying  out  practically  the  idea  suggested  in  Fig.  6,  by 
making  the  necessary  additions  to  the  apparatus  there  in- 
dicated. It  would  be  necessary  to  measure  the  relative 
luminosity  of  the  selected  spectral  hue  and  the  white  light, 
and  then  to  mix  them  in  proper  proportions,  till  the  mixture 
matched  the  colour  of  the  painted  paper,  etc.  The  second 
and  third  colour-constants  cart  be  more  easily  determined. 

It  may  be  well  here  to  refer  to  the  terms  used  to  indicate 
these  constants.  For  the  first  constant,  the  wordi  purity,  in 
the  sense  of  freedom  from  white  light  (or  from  the  sensa- 
tion of  white),  is  well  adapted.  The  term  luminosity  will 
be  employed  in  this  work  to  indicate  the  second  constant  ; 
the  third  constant  will  generally  be  referred  to  by  the  term 
hue.  Colours  are  often  also  called  intense,  or  saturated, 
when  they  excel  both  in  purity  and  luminosity  ; for  it  is 
quite  evident  that,  however  pure  the  coloured  light  may  be, 
it  still  will  produce  very  little  effect  on  the  eye  if  its  total 
quantity  be  small ; on  the  other  hand,  it  is  plain  that  its 
action  on  the  same  organ  will  not  be  considerable,  if  it  is 
diluted  with  much  white  light.  Purity  and  luminosity  are, 
then,  the  factors  on  which  the  intensity  or  saturation  de- 
pends. We  shall  see  hereafter  that  this  is  strictly  true  only 
within  certain  limits,  and  that  an  inordinate  increase  of 
luminosity  is  attended  with  a loss  of  intensity  of  hue  or 
saturation. 

Having  defined  the  three  constants  of  colour,  it  will  be 
interesting  to  inquire  into  the  sensitiveness  of  the  eye  in 
these  directions.  This  subject  has  been  studied  by  Aubert, 
who  made  an  extensive  set  of  observations  with  the  aid  of 
coloured  disks.*  It  was  found  that  the  addition  of  one 
part  of  white  light  to  360  parts  of  coloured  light  produced 
a change  which  was  perceptible  to  the  eye  ; smaller  amounts 
failed  to  bring  about  this  result.  It  was  also  ascertained 


* Aubert,  “ Physiologic  der  Netzhaut,”  Breslau,  1865. 


40 


MODERN  CHROMATICS. 


that  mingling  the  coloured  light  of  a disk  with  from  120  to 
180  j}arts  of  white  light  (from  white  paper)  caused  it  to  be- 
come imperceptible,  the  hue  being  no  longer  distinguishable 
from  that  of  the  paper.*  Differences  in  luminosity  as  small 
as  to  could  also,  under  favourable  circumstances,  be 
perceived.  It  hence  followed  that  irregularities  in  the  illu- 
mination, or  distribution  of  pigment  over  a surface,  which 
were  smaller  than  of  the  total  amount  of  light  reflected, 
could  no  longer  be  noticed  by  the  eye.  Experiments  with 
red,  orange,  and  blue  disks  were  made  on  the  sensitiveness 
of  the  eye  to  changes  of  hue  or  wave-length  ; thus,  the 
combination  of  the  blue  disk  with  a minute  portion  of  the 
red  disk  altered  its  hue,  moving  it  a little  toward  violet  ; 
on  reversing  the  case,  or  adding  a little  blue  to  the  red  disk, 
the  hue  of  the  latter  moved  in  the  direction  of  purple, f 
Similar  combinations  were  made  with  the  other  disks.  Au- 
bert  ascertained  in  this  way  that  recognizable  changes  of 
hue  could  be  produced,  by  the  addition  of  cpiantities  of 
coloured  light,  as  small  as  from  3-oir  total 

amount  of  light  involved.  From  such  data  he  calculated 
that  in  a solar  spectrum  at  least  a thousaml  distinguishable 
hues  are  visible.  But  we  can  still  recognize  these  hues, 
when  the  light  producing  them  is  subjected  to  considerable 
variation  in  luminosity.  lA't  us  limit  ourselves  to  100 
slight  variations,  which  we  can  produce  by  gradually  in- 
creasing the  brightness  of  our  spectrum,  till  it  finally  is 
five  times  as  luminous  as  it  originally  was.  This  will  fur- 
nish us  with  a hundred  thousand  hues,  differing  perceptibly 
from  each  other.  If  each  of  these  hues  is  again  varied 

* To  obtain  correct  results  it  is  of  course  necessary  to  know  the  lumi- 
nosity of  the  coloured  disk  as  compared  with  the  whit^disk,  for  in  the  above 
results  by  Aubert  they  are  considered  to  be  equal.  "With  the  aid  of  the 
table  of  luminosities  previously  given,  this  correction  can  be  made,  and  it 
will  be  found  that  four  or  five  times  as  much  white  light  is  actually  neces- 
sary as  is  indicated  above. 

f Compare  Chapter  x. 


THE  CONSTANTS  OF  COLOUK. 


41 


twenty  times,  by  the  addition  of  different  quantities  of 
white  light,  it  carries  the  number  of  tints  we  are  able  to 
distinguish  up  as  high  as  two  millions.  In  this  calculation 
no  account  is  taken  of  the  whole  series  of  purples,  or  of 
colours  which  are  very  luminous  or  very  dark,  or  mixed 
with  much  white  light. 

To  the  above  we  may  add  that  interesting  experiments 
on  the  sensitiveness  of  the  efe  to  the  different  spectral 
colours  have  also  been  made  by  Charles  Pierce,  who  found 
that  the  photometric  susceptibility  of  the  eye  was  the  same 
for  all  colours.  (See  “American  Journal  of  Science  and 
Arts,”  April,  1877.) 

With  the  aid  of  Vierordt’s  measurements  previously 
given,  and  the  determinations  by  the  author  of  the  spaces 
occupied  by  the  different  colours  in  the  spectrum,  a very 
interesting  point  can  now  be  settled,  viz.  : we  can  ascertain 
in  what  proportions  the  different  colours  are  present  in 
white  light.  The  amount  of  red  light,  for  example,  which 
is  present  will  evidently  be  equal  to  the  space  which  it  oc- 
cupies in  the  spectrum,  multiplied  by  its  luminosity,  and 
the  same  will  be  true  of  all  the  other  colours.  The  author 
constructed  a curve  representing  Vierordt’s  results,  and 
from  this,  taken  in  combination  with  his  own  determina- 
tions of  the  extent  of  the  coloured  spaces,  obtained  the 
following  table  : 


Table  showing  the  Amounts  of  Coloured  Light  in  1,000  Parts  of  White 

Sunlight. 


Red 64 

Orange-red 140 

Orange 80 

Orange-yellow 114 

Yellow 64 

Greenish-yellow 206 

Yellowish-green 121 

Green  and  blue- green 134 

Cyan-blue 32 


42 


MODERN  CHROMATICS. 


Blue 

Ultramarine  and  blue-violet 
Violet 


40 

20 

5 


1,000 


Artists  are  in  the  habit  of  dividing  up  colours  into  warm 
and  cold.  Kow,  if  we  draw  the  dividing  line  so  as  to  in- 
clude among  the  warm  colours  red,  orange-red,  orange, 
orange-yellow,  yellow,  greenish-yellow,  and  yellowish-green, 
then  in  ^^ilite  light  the  total  luminosity  of  the  warm  colours 
will  he  rather  more  than  three  times  as  great  as  that  of  the 
cold  colours.  If  we  exclude  from  the  list  of  warm  colours 
yellowish-green,  then  they  will  he  oidy  about  twice  as 
luminous  as  the  cold.  We  shall  make  use  of  this  table 
hereafter. 

It  may  have  seemed  strange  that  the  chrome-yellow 
paper  ])reviously  mentioned  rctlected  eighty  per  cent,  of 
light  (the  retlecting  ])ower  of  white  paper  being  lOD],  while 
the  table  just  given  states  that  white  light  contains  only  a 
little  more  than  live  })cr  cent,  of  ])iire  yellow  light.  It  will, 
however,  be  shown  in  a future  chapter  that  chrome-yellow 
really  reflects  not  only  the  pure  yellow  rays,  but  also  the 
orange-yellow  and  greenish-yellow,  besides  much  of  the  red, 
orange,  and  green  light.  I>y  mixture,  all  these  colours 
finally  make  a yellow,  as  will  be  exj)lained  in  Chapter  X. 
The  high  luminosity  of  some  of  the  other  coloured  paj)ers 
is  to  be  explained  in  a similar  manner. 


CHAPTER  IV. 


PRODUCTION  OF  COLOUR  BY  INTERFERENCE  AND 
POLARIZATION. 

In  Chapter  II.  we  studied  the  spectral  tints  produced  by 
a prism  and  by  a grating  ; these  were  found  to  be  pure  and 
brilliant  as  well  as  very  numerous,  and  consequently  were 
adopted  as  standards  for  comparison.  Most  nearly  allied 
to  these  central  normal  colours  are  those  which  are  pro- 
duced by  the  polarization  of  light.  In  this  class  we  meet 
with  a far  greater  variety  of  hues  than  is  presented  by  the 
solar  spectrum  ; and,  instead  of  a simple  arrangement  of 
delicately  shading  bands,  we  encounter  an  immense  variety 
of  chromatic  combinations,  sometimes  worked  out  with  ex- 
quisite beauty,  but  as  often  arranged  in  a strange  fantastic 
manner,  that  suggests  we  have  entered  a new  world  of 
colour,  which  is  ruled  over  by  laws  quite  different  from 
those  to  which  we  have  been  accustomed.  And  it  is  indeed 
so  ; the  tints  and  their  arrangement  depend  on  the  geo- 
metrical laws  which  build  up  the  crystal  out  of  its  mole- 
cules, and  on  the  retardation  which  the  waves  of  light  ex- 
perience in  sweeping  through  them,  so  that  in  the  colours 
of  polarization  we  see,  as  it  were.  Nature’s  mathematical 
laws  laying  aside  for  a moment  their  stiff  awkwardness,  and 
gayly  manifesting  themselves  in  play. 

The  apparatus  necessary  for  the  study  of  these  fasci- 
nating and  often  audacious  colour-combinations  is  not 
necessarily  complicated  or  very  expensive.  A simple  form 


44 


MODERN  CHROMATICS. 


of  polariscope  is  sliown  in  Fig.  11.  It  consists  merely  of 
a plate  of  window-glass  at  P,  which  is  so  placed  that  the 
angle,  a,  is  33°  as  nearly  as  possible  ; at  X is  a Xicol’s 
prism,  and  at  L a plano-convex  lens  with  a focal  length  ot 
about  an  inch.  The  distance  of  the  prism  from  the  plate 


of  glass  is  ten  inches;  tlie  lens  is  removable  at  pleasure, 
the  Xicol’s  ])rism  is  ca]>able  ot  revnlving  around  its  longei 
axis.  It’,  now,  light  from  a white  cloud  l)e  reflected  Irom 
the  plate  of  glass  toward  the  Xicol  s ]irism,  as  indicated  b\ 
the  arrow,  some  of  it  will  ordinarily  traverse  the  ])rism  and 
reach  the  eye  at  F ; the  prism  should  now  be  turned  till 
this  light  is  cut  off,  and  the  instrument  is  then  ready  for 
use.  Thin  slips  of  selenite  or  crystals  of  tartaric  acid 
placed  at  aS',  so  that  they  are  magnified,  display  the  colours 
of  polarization  very  beautifully.  The  arrangement  just 
described  constitutes  a simple  ])olarizing  microscope  ; if  a 
compound  polarizing  microsco))e  can  be  obtained,  it  will  be 
still  more  easy  to  study  the  colours  and  combination  of 
colours  presented  by  the  crystals  of  many  different  salts. 
By  dissolving  a grain  or  two  of  the  substance  in  a drop  of 
water,  and  allowing  it  to  crystallize  out  on  a slip  of  glass,  it 
is  possible  very  easily  to  make  objects  suitable  for  exami- 
nation. 

Thin  plates  of  selenite,  obtained  by  removing  successive 
layers  with  a penknife,  answer  admirably  if  we  wish  to 
study  the  phenomena  of  chromatic  polarization  in  their 
simplest  forms.  It  will  often  be  found  that  nearly  the 


PRODUCTION  OF  COLOUR  BY  INTERFERENCE,  ETC.  45 


whole  plate  presents  a single  unshaded  hue,  which  bears  a 
close  resemblance  to  a patch  of  colour  taken  from  some  por- 
tion of  the  spectrum.  But,  however  bright  the  colour  may 
be,  it  is  never  free  from  an  admixture  of  white  light,  and 
it  is  the  constant  presence  of  this  foreign  element  which 
causes  the  colours  of  polarization  to  appear  a little  less  in- 
tense than  those  of  the  spectri^m.  With  plates  which  are 
somewhat  thicker  the  proportion  of  white  light  increases, 
washing  out  the  colour,  till  only  a faint  reminiscence  of  it 
is  left.  The  more  powerful  tints,  however,  quite  equal  and 
probably  surpass  in  purity — that  is,  in  freedom  from  white 
light — the  most  intense  sunset  hues.  Among  these  colours 
we  find  many  shades  of  red  and  purple-red  ; all  the  red- 
orange  hues  are  represented,  and  the  same  is  true  of  the 
other  colours  found  in  the  spectrum.  Besides,  there  is  a 
range  of  purples  which  bridges  over  the  chasm  between  the 
violets  and  reds  ; faint  rose-tints  are  also  present  in  abun- 
dance, and  the  same  is  true  of  the  pale  greens  and  bluish- 
greens.  In  addition  to  this,  quite  thin  slips  furnish  a dis- 
tinct set  of  tints  which  are  peculiar  in  appearance,  and 
which,  when  once  seen,  are  never  forgotten  ; a singular 
tawny  yellow  will  be  noticed  in  combination  with  a bluish- 
grey  ; the  yellow,  such  as  it  is,  shading  into  an  orange 
nearly  allied  to  it,  and  this  again  into  a brick-red  ; black 
and  white  will  be  associated  with  these  subdued  tints  ; the 
general  impression  produced  by  these  combinations  being 
sombre  if  not  dreary. 

These  slips  of  selenite  furnish  neither  beautiful  nor  com- 
plicated patterns,  the  tints  being  for  the  most  part  arranged 
in  parallel  bands,  with  here  and  there  angular  patches,  often 
in  sharp  contrast  with  the  other  masses  of  colour.  There  is 
no  noticeable  attempt  at  chromatic  composition,  except  per- 
haps a little  along  fractured  edges,  where  we  frequently 
meet  with  pale  grey  or  white  deepening  into  a fox-coloured 
yellow,  followed  by  a red-violet,  brightening  into  a sea- 
green  dashed  with  pure  ultramarine,  or  changing  suddenly 


46 


MODERN  CHROMATICS. 


into  a full  orange-yellow,  after  which  may  follow  a broad 
held  of  purple.  Just  as  often  all  the  tints  are  pale,  like 
those  used  on  maps,  with  a narrow  fringe  on  the  edge,  of 
rich  variegated  hues.  The  colour-combinations  seldom  rise 
into  great  beauty,  though  they  often  astonish  and  dazzle 
by  their  audacity  and  total  disregard  of  all  known  laws  of 
chromatic  composition.  The  brilliancy  and  purity  of  these 
tints  are  so  great,  and  they  are  laid  on  with  such  an  unfal- 
tering hand,  that  all  these  wild  freaks  are  performed  com- 
paratively with  impunity,  and  it  is  only  when  we  proceed 
to  make  copies  of  these  strange  designs  that  we  become 
fully  aware  of  their  ]jeculiaritics,  and,  from  an  artistic  point 
of  view,  })ositive  defects. 

Crystals  of  tartaric  aci<l  ])resent  phenomena  which  are 
quite  diifercnt:  here  the  ])atterns are  rich  and  often  beautiful; 
the  colour  is  full  of  gra<lation,  touched  on  and  retouched 
and  wrought  out  with  ])atience  in  delicate,  complicated 
forms,  which  echo  or  faintly  oppose  the  grand  ruling  ideas 
of  the  composition.  We  may  have  a wonderfully  sha])ed 
mass  radiating  in  curved  lines  over  the  entire  field,  tinte<l 
with  soft  grey  and  pale  yellow,  with  here  and  there  dashes 
of  colour  like  the  spots  on  a peacock’s  tail,  glowing  like 
coals  of  fire  ; all  this  being  set  off  by  very  dark  shades  of 
olive-green,  dark  browns  and  greys.  If  the  crystals  are  thin 
this  is  their  appearance  ; but  as  the  thickness  increases  so 
does  the  brilliancy  of  the  hues,  which  are  sure  to  be  well 
contrasted  with  large  masses  of  deep  shade.  The  soft  gra- 
dations, the  shar])  contrasts,  the  brilliant  and  pale  colours, 
the  dark  shadows  and  the  wonderful  forms,  all  combine  to 
lend  to  these  pictures  a peculiar  charm  which  is  not  wholly 
lost  even  in  copies  executed  in  ordinary  pigments. 

Common  sugar,  if  allowed  time  to  crystallize  out  slowly, 
furnishes  appearances  somewhat  resembling  the  above,  but 
the  designs  are  more  formal  and  less  interesting.  Crystals 
of  nitrate  of  potash  present  appearances,  again,  which  are 
totally  unlike  those  above  mentioned  ; here  we  have  a great 


PRODUCTION  OF  COLOUR  BY  INTERFERENCE,  ETC.  47 


number  of  delicately  tinted  threads  of  light ; there  will  be 
purples  and  golden  greens  or  dull  olive-greens  and  carmines, 
woven  together  so  closely  as  almost  to  produce  a neutral 
tint,  which  will  brighten  suddenly  and  display  combinations 
of  purple-red  with  green,  dashed  here  and  there  with  pure 
ultramarine.  These  tinted  threads  of  light  will  be  disj)osed 
with  regularity  as  though  it  had  been  intended  to  weave 
them  into  some  wonderful  cashmere-like  pattern,  and  then 
warp  and  woof  had  been  suddenly  abandoned  and  for- 
gotten. 

It  would  be  useless  to  multiply  these  descriptions — every 
salt  has  its  own  peculiarities  and  suggests  its  own  train 
of  fancies  ; some  glow  like  coloured  gems  with  polished 
facets,  or  bristle  with  golden  spears  like  the  advancing  ranks 
of  two  hosts  in  conflict,  or  suggest  a rich  vegetation  made 
of  gold  and  jewels  and  bathed  in  sunset  hues.  Artists  who 
see  these  exhibitions  for  the  first  time  are  generally  very 
much  impressed  by  their  strange  beauty,  and  not  unfre- 
quently  insist  that  their  range  of  colour-conceptions  has 
been  enlarged.  It  has  often  seemed  to  the  author  that  the 
cautious  occasional  study  of  some  of  these  combinations 
might  be  useful  to  the  decorator  in  suggesting  new  concep- 
tions of  the  possibilities,  within  his  reach. 

When  polarized  light  is  made  to  traverse  crystals  in  the 
direction  of  their  optic  axes,  phenomena  of  a different  kind 
are  presented.  They  were  discovered  in  1813  by  Brewster, 
and,  on  account  of  their  scientific  interest  and  a certain 
beauty,  have  since  then  greatly  attracted  the  attention  of 
physicists  and  even  of  mathematicians.  A series  of  rainbow- 
like hues,  disposed  in  concentric  circles,  is  seen  on  a white 
field  ; a dark-grey  cross  is  drawn  across  the  gayly  coloured 
circles,  and,  after  dividing  them  in  four  quadrants,  fades 
out  in  the  surrounding  white  field.  By  a slight  change  in 
the  adjustment  of  the  apparatus,  the  grey  cross  can  be  made 
white  ; the  rings  then  assume  the  complementary  tints. 
Other  crystals,  again,  furnish  double  sets  of  rings,  the  dark 


48 


MODERN  CHROMATICS. 


cross  being  shared  by  them  jointly,  or  so  altered  in  form  as 
no  longer  to  be  recognizable. 

These  appearances  have  been  considered  by  many  phys- 
icists to  be  extraordinarily  beautiful ; it  is,  however,  to  be 
suspected  that  in  this  case  the  judgment  was  swayed  by 
other  considerations  than  those  of  mere  beauty.  The  rarity 
of  the  phenomenon,  the  difficulty  of  exhibiting  it,  the  bril- 
liant list  of  names  identified  with  it,  along  with  the  insight 
it  furnishes  as  to  the  molecular  constitution  of  crystals,  all 
combine  to  warp  the  judgment,  and  to  seriously  influence 
its  final  award.  In  point  of  fact,  the  formal  nature  of  the 
figures,  the  constant  repetition  of  the  rainbow-tints  in  the 
same  set  order,  which  is  that  of  the  spectrum,  both  exclude 
the  possibility  of  the  charming  colour-combinations  so  fre- 
quently presented  by  many  salts  when  simply  crystallized 
on  a slip  of  glass.  The  cross  and  rings  are  not  for  a mo- 
ment, in  matter  of  beauty,  to  be  compared  with  the  appear- 
ances presented  by  crystals  of  tartaric  acid. 

Glass  which  has  been  heated  and  then  suddenly  cooled, 
or  glass  which  is  under  strain,  exhibits  phenomena  of  colour 
closely  related  to  the  above  ; wo  have  as  it  were  a set  of 
distorted  crosses  and  rings  which  sometimes  lend  themselves 
more  kindly  to  the  production  of  chromatic  elTects  than  is 
the  case  with  the  normal  figures. 

In  ordinary  life  the  colours  of  polarization  are  never 
seen  ; the  fairy  world  where  they  reign  cannot  be  entered 
without  other  aid  than  the  unassisted  eye.  This  is  not  a 
matter  for  regret  ; the  ]mrity  of  the  hues  and  the  audacious 
character  of  their  combinations  cause  their  gayety  to  appear 
strange  and  unnatural  to  eyes  accustomed  to  the  far  more 
sombre  hues  appropriate  to  a world  in  which  labour  and 
trouble  are  such  important  and  ever-present  elements.  The 
colours  even  of  flowers  have  a thoughtful  cast,  when  com- 
pared with  those  of  polarization. 

The  colours  which  have  just  been  considered  are  pro- 
duced in  a peculiar  manner  ; the  complete  explanation  is 


PRODUCTION  OF  COLOUR  BY  INTERFERENCE,  ETC.  49 

long  and  tedious,  and  has  for  us  no  particular  interest.  The 
main  idea,  however,  is  this  : white  light  is  acted  on  in  such 
a way  that  one  of  its  constituents  is  suppressed  ; the  result 
is  coloured  light.  For  example,  if  we  strike  out  from  white 
light  the  yellow  rays,  what  remains  will  produce  on  us  the 
sensation  of  blue  ; if  we  cut  off  the  green  rays,  the  remain- 
der will  appear  purple.  The  'reason  of  this  will  be  more 
fully  appreciated  after  a study  of  the  facts  presented  in 
Chapters  XI.  and  XII.  To  effect  this  sifting  out  of  certain 
rays  a polarizing  apparatus  is  employed  ; when  the  crystals 
are  removed  from  it,  the  colour  instantly  vanishes.  Xow, 
it  so  happens  that  there  is  a class  of  natural  objects  capable 
of  displaying  exactly  the  same  hues  without  the  intervention 
of  any  piece  of  apparatus — all  objects  that  fulfill  a certain 
condition  may  be  reckoned  in  this  class  ; it  is  merely  that 
their  thickness  should  be  very  small.  Thin  layers  of  water, 
air,  glass,  of  metallic  oxides,  of  organic  substances,  in  fact 
of  almost  everything,  display  these  colours.  The  most  fa- 
miliar example  is  furnished  by  a soap-bubble.  When  it  first 
begins  to  grow,  it  is  destitute  of  colour  and  perfectly  trans- 
parent ; it  gives  by  reflection  from  its  spherical  surface  a 
distorted  image  of  the  window,  with  the  bars  all  curved, 
but  no  unusual  hues  are  noticed  till  it  has  become  somewhat 
enlarged.*  Then  faint  greens  and  rose-tints  begin  to  make 
their  appearance,  mingling  uneasily  together  as  if  subjected 
to  a constant  stirring  process.  As  the  bubble  expands  and 
the  film  becomes  more  attenuated,  the  colours  gain  in  bril- 
liancy, and  a set  of  magnificent  blue  and  orange  hues,  pur- 
ples, yellows  and  superb  greens  replaces  the  pale  colours 
which  marked  the  early  stages,  and  by  their  changing  flow 
and  perpetual  play  fascinate  the  beholder.  If  the  bubble 

It  is  not  very  uncommon  to  meet  with  paintings  in  which  a bubble 
has  been  represented  with  window-bars  on  its  surface,  where  nothing  of 
the  kind  could  have  been  visible.  A friend  has  mentioned  to  the  author 
four  cases  where  different  artists  have  introduced  window-bars  instead  of 
sky  and  landscape,  on  the  surfaces  of  bubbles  which  were  in  the  open  air. 


50 


MODEKX  CHROMATICS. 


has  the  rare  fortune  to  live  to  a good  old  age,  at  its  upper 
portion  a different  series  of  tints  begins  to  be  developed  ; 
the  tawny  yellow,  before  mentioned,  begins  to  be  seen  in 
irregular  patches,  floating  around  among  the  more  brilliant 
hues,  a sign  that  the  attenuation  has  nearly  reached  its  ex- 
treme limit  ; but,  if  by  some  unusual  chance  it  should  be  a 
Methuselah  among  bubbles,  pale  white  and  grey  tints  also 
are  seen,  after  which  it  is  sure  to  burst.  A long-lived  soap- 
bubble  displays  every  colour  which  can  l)e  produced  by 
polarization.  The  thin  him  has  a sifting  action  on  white 
light,  which  in  its  final  result  is  the  same  as  in  the  case  of 
the  production  of  colour  by  ])olarized  light  : certain  rays  are 
struck  out,  and,  as  before,  white  light  deprived  of  one  of 
its  constituents  furnishes  coloured  light.  This  elimination 
is  accomplished  by  the  interference  of  the  waves  of  light  in- 
volved ; hence,  colours  })roduced  in  this  way  are  called  “ in- 
terference colours.”  The  colours  of  polarization  are  also  just 
as  truly  interference  colours,  but  they  are  not  usually  known 
under  this  name.  From  all  this  it  follows  that  the  colours 
produced  by  thin  layers,  or  by  very  fine  particles,  always 
contain  some  white  light,  and  consequently  cannot  quite 
rival  in  purity  or  intensity  the  si)ectral  lines. 

The  colours  of  iiolarization,  as  we  have  seen,  are  never 
met  with  outside  of  the  lalioratory.  Nature,  on  the  other 
hand,  here  and  there  with  a sparing  hand,  displays  in  small 
quantity,  and  as  a rarity,  the  colours  of  interference.  They 
are  used  as  a wonderful  kind  of  jewelry  in  the  adornment 
of  many  birds  ; lavishly  so  in  the  case  of  the  common  pea- 
cock, where  the  breast  and  tail  feathers  in  full  sunshine  dis- 
play flashing,  dazzling  hues,  which  make  our  artificial  or- 
naments appear  pale  and  tame.  In  contemplating  these 
astonishing  hues,  or  those  of  that  tiny  winged  jewel,  the 
humming-bird,  we  are  struck  by  the  circumstance  that  they 
actually  have  a metallic  brilliancy,  which  we  in  vain  at- 
tempt to  rival  with  our  brightest  pigments.  To  compete 
with  them  successfully,  it  is  necessary  to  substitute  a sur- 


PRODUCTIOX  OF  COLOUR  BY  INTERFERENCE,  ETC.  51 

face  of  silver  for  the  white  paper,  and  to  cover  it  with  the 
purest  and  most  transparent  glazes.  This  appearance  of 
metallic  lustre  depends  on  the  circumstance  that  much  col- 
oured light  is  reflected,  mingled  with  only  a small  quantity 
of  white  light,  the  great  bulk  of  the  latter  being  absorbed 
by  the  dark  pigment  contained  in  the  interior  of  the  feath- 
ers. When  this  dark  pigment  is  absent,  we  have  as  before 
colour  ; but,  being  mingled  with  much  reflected  white  light, 
it  presents  simply  an  appearance  like  that  of  mother-of- 
pearl. 

There  is  yet  another  peculiarity  of  the  colours  now 
under  consideration,  which  still  more  completely  separates 
them  from  the  hues  furnished  by  pigments  : it  is  their  va- 
riability. These  colours,  as  has  been  mentioned,  are  pro- 
duced by  the  interference  of  the  waves  of  light  which  are 
reflected  from  the  thin  films  : the  nature  of  this  interference 
depends  partly  on  the  angle  at  which  this  reflection  takes 
place,  so  that,  as  we  turn  a peacock’s  feather  in  the  hand, 
its  colour  constantly  changes.  The  same  is  true  of  the  tints 
of  the  soap-bubble,  and  of  interference  colours  in  general 
— the  hue  changes  wdth  the  position  of  the  eye  ; as  they  are 
viewed  more  and  more  obliquely,  the  tint  changes  in  the 
order  of  the  spectrum,  viz.,  from  red  to  orange,  to  yellow, 
etc. 

The  brilliant  metallic  colours  exhibited  by  many  insects, 
particularly  the  beetles,  belong  also  in  this  class,  so  also  the 
more  subdued  steel-blues  and  bottle-greens  displayed  by 
many  species  of  flies.  So  commonly  does  this  occur  that  it 
suggests  the  idea  that  these  humble  creatures  are  not  desti- 
tute of  a sense  for  colour  capable  of  gratification  by  bril- 
liant hues.  If  we  descend  into  the  watery  regions  we  find 
their  inhabitants  richly  decorated  with  colours  of  the  same 
general  origin,  the  pearly  rainbow  hues  which  they  display 
all  depending  on  the  interference  of  light.  The  same  is 
true  of  the  iridescent  hues  which  so  commonly  adorn  shells 
externally  and  internally.  In  this  case  candour  compels  one 


52 


MODERX  CHROMATICS. 


to  admit  that  the  colours,  beautiful  as  they  are,  can  hardly 
be  a source  of  pleasure  to  the  occupants  or  to  their  friends. 

Leaving  the  animated  world,  we  find  the  colours  of  in- 
terference shown  frequently,  but  in  an  inconspicuous  man- 
ner, by  rather  old  window-glass  ; some  of  the  alkali  seems 
to  be  removed  by  the  rain,  and  in  the  course  of  time  a thin 
■film  of  silica  capable  of  generating  these  hues  is  formed. 
In  antique  glass  which  has  long  remained  buried  this  pro- 
cess is  carried  much  further,  so  that  sometimes  the  whole 
plate  or  vase  tends  to  split  up  into  Hakes.  Here,  owing  to 
successive  reflections  on  many  layers,  the  light  which  reach- 
es the  eye  is  quite  bright,  and  the  colours  intense.  Crim- 
son, azure,  and  gold  are  found  in  combination  ; blue  melts 
into  purple  or  Hashes  into  red  ; ruby  tints  contrast  with 
emerald  hues  : each  change  of  the  j)Osition  of  the  eye  or  of 
the  direction  of  the  light  gives  rise  to  a new  and  startling 
effect.  In  other  cases  broad  Helds  of  colour,  with  much 
gentle  gradation  and  mingling  of  tender  pearly  hues,  re- 
place the  gorgeous  ])risrnatic  tints,  and  fascinate  the  be- 
holder with  their  soft  brilliancy. 

The  iridescent  hues  of  many  minerals  fall  into  the  same 
general  class  ; they  are  beautifully  displayed  by  some  of 
the  feldspars,  and  the  brilliant  hues  found  on  anthracite 
coal  have  also  the  same  origin.  The  blue  lilms  often  pur- 
posely produced  on  steel  are  due  to  thin  layers  of  oxide  of 
iron  which  suppress  the  yellow  rays.  Other  cases  might  be 
mentioned,  but  these  will  suHice  for  the  present. 


CHAPTER  V. 


ON  THE  COLOURS  OF  OPALESCENT  MEDIA. 

If  white  light  be  allowed  to  fall  on  water  which  is  con- 
tained in  a clear,  colourless  glass  vessel,  some  of  it  will  be 
reflected  from  the  surface  of  the  liquid,  while  another  por- 
tion will  traverse  the  water  and  finally  again  reach  the  air. 
These  well-known  facts  are  represented  in  Fig.  12.  An 
eye  placed  at  E will  perceive  the  reflected  light  to  be  white, 
and  the  transmitted  light  will  also  appear  white  to  an  eye 
situated  at  O.  But,  if  now  a little  milk  be  added  to  the 
water,  a remarkable  change  will  be  produced : light  will,  as 
before,  be  reflected  from  the  surface  to  the  eye  placed  at 
and  this  surface-light  will  still  be  white  ; but  the  little  milk- 
globules  under  the  surface  and  throughout  the  liquid  will 
also  reflect  light  toE — this  light  will  be  bluish.  From  this 
experiment,  then,  it  appears  that  the  minute  globules  sus- 
pended in  the  liquid  have  the  power  of  reflecting  light  of  a 
bluish  tint.  In  Fig.  12  the  light  is  represented  as  being 
reflected  only  in  one  direction  ; but,  when  the  milk-globules 
are  added,  they  scatter  reflected  light  in  many  directions,  so 
that  an  eye  placed  anywhere  above  the  liquid  perceives  this 
bluish  appearance. 

On  the  other  hand,  after  the  addition  of  the  milk,  the 
light  at  O (Fig.  12),  which  has  passed  through  the  milky 
liquid,  will  be  found  to  have  acquired  a yellowish  tint. 
From  this  it  appears  that  fine  particles  suspended  in  a liquid 
have  the  power  of  dividing  white  light  into  two  portions, 
tinted  respectively  yellowish  and  bluish.  If  more  milk  be 


54 


MODERN  CHROMATICS. 


added  to  the  water,  white  light  will  mingle  in  and  will  final- 
ly overpower  the  bluish  reflected  light,  so  that  it  will  hardly 
be  noticed  ; as  the  quantity  of  milk  is  increased,  the  colour 
of  the  transmitted  light  will  pass  from  yellow  to  orange,  to 
red,  and  finally  disappear,  the  liquid  having  become  at  last 
so  opaque  as  to  cease  to  transmit  light  altogether. 


Fio.  12.— Reflection  and  Transmission  of  Light  by  Water. 


This  very  curious  action  is  not  confined  to  mixtures  of 
milk  and  water,  but  is  exhibited  whenever  very  fine  parti- 
cles are  suspended  in  a medium  dilferent  from  themselves. 
If  an  alcoholic  solution  of  a resin  is  poured  with  constant 
stirring  into  water,  very  fine  particles  of  resin  are  left  sus- 
pended in  the  liquid,  and  give  rise  to  the  appearances  just 
described.  Brllcke  dissolves  one  part  of  mastic  in  eighty- 
seven  parts  of  alcohol,  and  then  mixes  with  water,  the  water 


ON  THE  COLOURS  OF  OPALESCENT  MEDIA. 


55 


being  kept  in  constant  agitation.  A liquid  prepared  in  this 
way  shows  by  reflected  light  a soft  sky-like  hue,  the  colour 
of  the  light  which  has  passed  through  being  either  yellow 
or  red,  according  to  the  thickness  of  the  layer  traversed. 
The  suspended  particles  of  resin  are  very  fine,  and  remain 
mingled  with  the  water  for  months  ; they  are  often  so  fine 
as  to  escape  detection  by  the  most  powerful  microscopes. 

Some  kinds  of  glass  which  are  used  for  ornamental  pur- 
poses possess  the  same  property,  appearing  bluish-white  by 
reflected  light,  but  tingeing  the  light  which  comes  through 
them  red  or  orange-red.  The  beautiful  tints  of  the  opal 
probably  have  the  same  origin,  and  the  same  is  true  also  of 
the  bluish,  milky  colour  which  characterizes  many  other 
varieties  of  quartz. 

Not  only  liquids  and  solids  exhibit  this  phenomenon  of 
opalescence,  but  we  find  it  also  sometimes  displayed  else- 
where ; thus,  for  example,  a thin  column  of  smoke  from 
burning  wood  reflects  quite  a proportion  of  blue  light, 
while  the  sunlight  which  traverses  it  is  tinted  of  a brown- 
ish-yellow, or  it  may  be,  even  red,  if  the  smoke  is  pretty 
dense. 

All  these  phenomena  are  probably  due  to  an  interference 
of  light,  which  is  brought  about  by  the  presence  of  the  fine 
particles,  the  shorter  waves  being  reflected  more  copiously 
than  those  which  are  longer  ; these  last,  on  the  other  hand, 
being  more  abundantly  transmitted.  An  elaborate  expla- 
nation of  the  mode  in  which  the  interference  takes  place 
would  be  foreign  to  the  purpose  of  the  present  work  ; we 
therefore  pass  on  to  the  consideration  of  the  more  practical 
aspects  of  this  matter.* 

It  will  be  well  to  notice,  in  the  first  place,  certain  con- 
ditions which  favour  not  so  much  the  formation  as  the  per- 
ception of  the  tints  in  question  : thus  it  will  be  found  that 

* Compare  E.  Briicke,  in  Poggendorff ’s  “ Annalen,”  Bd.  88,  S.  363  ; 
also  Bezold’s  “Farbenlehre,”  p.  89. 


56 


MODERN  CHROMATICS. 


the  blue  tint,  in  the  experiments  with  the  liquids,  is  best 
exhibited  by  placing  the  containing  glass  vessel  on  a black 
surface.  This  effectually  prevents  the  blue  reflected  light 
from  being  mingled  with  rays  which  have  been  directly 
transmitted  from  underneath.  Indeed,  the  mere  presence 
or  absence  of  a dark  background  may  cause  the  tint  which 


Fio.  IS.— Smoke  appears  Blue  on  a Dark  Background ; Brown  on  a Light  Background. 

finally  is  perceived  by  the  eye  to  change  from  yellow  to 
blue,  the  other  conditions  all  remaining  unaltered.  Thus  in 
Fig.  13  we  have  thin  smoke  seen  partly  against  a dark 
background,  and  partly  against  a sky^’overed  with  white 
clouds : the  lower  portion  is  blue,  from  reflected  light, 
while  in  the  upper  portion  this  tint  is  overpowered  by  the 
greater  intensity  of  the  transmitted  light,  which  is  yellow- 
ish-brown. As  a general  thing,  the  reflected  and  transmit- 
ted beams  are  both  present  ; dark  backgrounds  favour  the 
former,  luminous  ones  the  latter. 

If  a thin  coating  of  white  paint,  such  as  white  lead  or 
zinc-white,  is  spread  over  a black  or  dark  ground,  the 
touches  so  laid  on  will  have  a decidedly  bluish  tint,  owing 
to  the  causes  which  have  just  been  considered.  If  a draw- 
ing on  dark  paper  be  retouched  with  zinc-white  used  as  a 
water-colour,  the  touches  will  appear  bluish  and  inharrno- 
nious,  unless  especial  care  is  taken  to  prevent  the  white 


ON  THE  COLOUKS  OF  OPALESCENT  MEDIA. 


57 


pigment  from  being  to  some  extent  translucent ; this  disa- 
greeable appearance  can  only  be  prevented  by  making  each 
touch  dense  and  quite  opaque.  For  the  production  of  such 
effects  it  is  not  even  necessary  to  go  through  the  formality 
of  laying  the  white  pigment  on  a dark  ground  ; white  lead 
mingled  with  any  of  the  ordinary  black  pigments  gives  not 
a pure  but  a bluish  grey.  Tl^is  is  very  marked  in  the  case 
of  black  prepared  from  cork,  which  hence  has  sometimes 
been  called  “beggars’  ultramarine.”  If  yellow  pigments 
are  mixed  with  black,  the  effect  is  not  simply  to  darken  the 
yellow,  as  would  be  expected,  but  it  is  converted  into  an 
olive-green.  This  is  particularly  the  case  with  those  pig- 
ments which  approximate  to  pure  yellow  in  hue,  such  as 
gamboge  and  aureolin  ; the  least  admixture  of  dark  pig- 
ment carries  them  over  toward  green.  But  if  these  black- 
and-white  or  black-and-yellow  pigments  are  combined  by 
the  method  of  rotating  disks  (see  Chapter  X.),  we  obtain 
pure  greys  or  darker  yellow  tints,  showing  that  the  blue 
hue  is  not,  as  many  suppose,  inherent  in  the  black  pigment, 
but  an  accident  due  to  the  mode  of  its  employment.  The 
above-mentioned  cases  are  marked  examples  of  the  applica- 
tion of  these  principles  to  painting  ; but  in  a more  subtile 
way  the  whole  theory  of  the  process  of  oil-painting  takes 
cognizance  of  them,  and  is  so  adjusted  as  to  avoid  difficul- 
ties thus  introduced,  or,  more  rarely,  to  utilize  them.  It  is 
perhajDS  scarcely  necessary  to  add  that  the  somewhat  bluish 
tone  of  drawings  made  with  body  colour,  or  of  frescoes,  is 
due  to  these  same  causes. 

Having  hinted  at  the  influence  which  this  peculiar  op- 
tical action  exerts  on  the  infancy  of  a picture,  we  pass  on 
to  consider  some  of  its  effects  on  a painting  in  its  old  age. 
It  is  well  known  that  old  oil-paintings  frequently  become 
more  or  less  covered  by  what  seems  to  be  a coating  of  grey 
or  bluish-grey  mould,  which,  spreading  itself  particularly 
over  the  darker  portions,  obscures  them,  so  that  all  details 
are  lost,  and  the  work  of  the  artist  entirely  destroyed.  In- 


58 


MODERX  CHROMATICS. 


vestigation  lias  shown  that  this  trouble  is  caused  by  an  im- 
mense number  of  fine  cracks  in  the  painting  itself,  which 
seem  to  act  somewhat  in  the  same  Avay  as  the  mixtures 
which  have  just  been  considered,  so  that  the  obseiwer  is 
practically  looking  at  the  picture  through  a rather  dense 
haze.  By  filling  these  invisible  cracks  up  with  varnish  the 
matter  is  someivhat  helped,  but  much  more  perfectly  by  the 
“regenerations  process”  of  Pettenkofer.  This  celebrated 
chemist  once  by  accident  used  an  old,  worn-out  oil-cloth 
mat,  from  which  the  pattern  had  mostly  disappeared,  to 
cover  a beaker  containing  hot  alcohol.  On  removing  the 
mat,  some  hours  afterward,  he  was  surprised  to  find  that 
the  portion  acted  on  by  tlie  alcoholic  vapours  had  been  reju- 
venated and  the  pattern  restored.  It  was  soon  ascertained 
that  the  vapours  had  softened  the  pigment,  and  the  separated 
grains  had  again  been  fused  together.  Exj^eriments  on  old 
oil-paintings  gave  similar  results,  and  the  process  is  now  in 
use  in  some  of  the  largest  Euroj)can  galleries. 

In  many  other  objects  besides  those  that  hai^e  been 
mentioned  these  peculiar  tints  can  be  observed  ; among 
minor  examples  may  be  mentioned  the  bluish-grey  or  green- 
ish-grey tint  Avhich  marks  the  course  of  veins  under  the 
skin  ; the  blue  or  greenish  colour  of  the  human  eye  also 
owes  its  tint  to  the  same  cause.  In  these  cases  an  opalescent 
membrane  is  spread  over  a dark  background,  and  the  colour 
is  produced  in  the  same  manner  as  in  the  experiments  de- 
scribed at  the  commencement  of  this  chapter.  In  blue  eyes 
there  is  no  real  blue  colouring-matter  at  all. 

It  is,  however,  the  sky  that  exhibits  this  class  of  tints 
on  the  grandest  scale,  as  well  as  in  the  greatest  perfection. 
Our  atmosphere,  even  Avhen  perfectly  clear,  contains  sus- 
pended in  it  immense  numbers  of  very  fine  particles  which 
never  sjettle  to  the  earth,  and  which  the  rain  has  no  power 
to  wash  down.  When  they  are  illuminated  by  sunlight 
they  reflect  white  light  mixed  with  a certain  proportion  of 
blue,  and  this  blue  is  seen  on  a black  background,  Avhich  is 


ON  THE  COLOURS  OF  OPALESCENT  MEDIA. 


59 


nothing  less  than  the  empty  space  in  which  the  earth  is 
hung.  Hence,  the  blue  colour  of  the  sky.  This  tint  on 
clear  days  can  be  traced  up  tolerably  near  the  sun,  indeed, 
until  the  brightness  of  the  sky  begins  to  be  blinding.  An 
examination  of  the  deepest  blue  portions  of  the  sky  with 
the  spectroscope  reveals  the  presence  of  much  white  light, 
so  that  the  blue  colour  is  very  far  from  being  pure  or  satu- 
rated— a fact  that  young  landscape-painters  soon  have 
forced  on  their  attention.  In  clear  weather,  as  long  as  the 
sun  is  at  a considerable  distance  from  the  horizon,  the  yel- 
low colour  which  accompanies  the  transmission  of  light 
through  an  opalescent  medium  is  not  much  noticed  ; but,  as 
the  sun  sinks  lower,  its  rays  traverse  an  always  increasing 
thickness  of  the  atmosphere,  and  encounter  a greater  num- 
ber of  fine  particles,  so  that  the  transmitted  light,  late  in 
the  afternoon,  becomes  decidedly  yellow  or  rather  orange- 
yellow. 

Having  thus  briefly  considered  the  production  of  or- 
dinary sky-tints,  let  us  pass  to  the  aspects  assumed  by  a 
landscape  under  the  influence  of  the  minute  suspended  par- 
ticles. These  atoms  will  of  course  reflect  light  toward  the 
observer,  and  this  light  will  add  itself  to  that  which  comes 
regularly  from  objects  in  the  landscape,  producing  thus 
important  changes  in  their  appearance.  The  very  thick 
layer  of  air  intervening  between  the  observer  and  the  most 
distant  mountains  will  send  to  the  eye  a very  large  amount 
of  whitish-blue  light,  which  will  not  greatly  differ  from  a 
sky- tint.  This  will  entirely  overpower  the  somewhat  feeble 
light  reflected  from  portions  of  the  mountain  in  shade,  so 
that  as  a result  we  shall  have  all  the  shadows  of  the  moun- 
tain represented  by  more  or  less  pure  sky-tints,  and  these 
tints  will  be  far  more  luminous,  far  brighter,  than  the  ori- 
ginal shadows  were.  No  details  will  be  visible  in  these 
wonderfully  shaped  bluish  patches.  Those  portions  of  the 
mountains,  on  the  other  hand,  which  are  in  full  sunlight  will 
still  visibly  send  light  to  the  e^m  through  the  haze,  and 


60 


MODERN  CHROMATICS. 


their  prevailing  tint  will  be  yellowish  or  orange,  or  some 
other  warm  tone.  Not  many  details  will  be  visible,  and  the 
actual  colours  or  local  colours  of  the  mountain  will  not  at 
all  appear,  or  at  most  will  only  blend  themselves  with  the 
soft,  warm  tints  due  to  the  medium.  In  a word,  the  con- 
trast between  the  light  and  shade  will  be  enormously  dimin- 
ished, so  that  the  general  luminosity  of  the  mountain  will 
be  hardly  less  than  that  of  the  sky  itself,  and  its  colour  will 
be  worked  out  mainly  in  tints  which  have  the  same  origin 
and  character  with  those  of  the  sky.  As  we  approach 
nearer  the  mountain,  these  effects  begin  slowly  to  diminish, 
and  in  the  sunlit  portions  delicate  greens,  varied  and  soft 
greys,  begin  to  make  their  appearance,  while  the  shadows 
lose  their  heavenly  blue,  and,  darkening,  become  bluish- 
grey.  Afterward  all  those  parts  lying  in  sunlight  display 
their  local  tints,  somewhat  softened,  and  the  coloured  light 
from  the  shadows  begins  to  make  itself  felt,  and,  mingling 
with  the  blue  reflected  light,  to  produce  soft  purplish-greys, 
greenish-greys,  and  other  nameless  tints.  The  sunlit  por- 
tions of  the  pine-trees  will  be  of  an  olive-green  or  of  a low 
greenish-yellow,  the  shadows  on  the  same  trees  being  pure 
grey  or  bluish-grey  without  any  suggestion  of  green.  On 
nearer  objects  these  effects  arc  less  traceable,  and  the  natural 
relation  between  light  and  shade  more  and  more  preserved, 
so  that  contrast  of  this  last  kind  becomes  progressively 
stronger  as  we  turn  from  the  most  distant  to  the  nearest 
objects.  All  these  effects  are  readily  traceable  on  any  or- 
dinary clear  day  ; they  change,  of  course,  with  the  condition 
of  the  atmosphere,  and  as  it  becomes  misty  the  blue  reflected 
light  changes  to  grey,  the  transmitted  light  not  being 
equally  affected. 

Late  in  the  afternoon,  when  the  sim  is  low,  its  rays  trav- 
erse very  thick  layers  of  the  atmosphere,  and  wonderful 
chromatic  effects  are  produced.  Near  the  sun  the  transmit- 
ted light  is  yellowish,  but  so  bright  that  the  coloui'  is  not 
very  perceptible  ; to  the  right  and  left  the  colour  deepens 


ON  THE  COLOURS  OF  OPALESCENT  MEDIA. 


61 


into  an  orange,  often  into  a red,  which,  as  the  distance  from 
the  sun  increases,  fades  out  into  a purplish-grey,  greyish- 
blue,  passing  finally  into  a sky-blue.  The  warm  tints  are 
produced  mainly  by  transmitted  light,  the  cold  ones  by  re- 
flected light,  and  the  neutral  hues  by  a combination  of  both. 
Above  the  sun  there  is,  in  clear  sunsets,  a rather  regular 
transition  upward,  from  the  colours  due  to  transmitted,  to 
those  produced  by  reflected  light.  As  the  sun  sinks  lower 
its  rays  encounter  a greater  mass  of  suspended  particles, 
and  the  warm  tints  above  mentioned  move  toward  the  red 
end  of  the  spectrum,  and  also  gain  in  intensity.  The  pres- 
ence of  clouds  breaks  up  the  symmetry  of  these  natural 
chromatic  compositions,  and  gives  rise  to  the  most  magnifl- 
cent  effects  of  colour  with  which  we  are  acquainted.  The 
landscape  itself  sympathizes  with  the  sky,  and  near  the  sun, 
chameleon-like,  assumes  an  orange  or  even  red  hue  ; while  at 
greater  distances  its  cold  tints  are  warmed,  even  the  greens 
being  changed  into  olive  or  yellowish  hues.  Simultaneous- 
ly the  shadows  lengthen  enormously,  bringing  thus  the  com- 
position into  grand  and  imposing  masses,  and  investing  even 
the  most  commonplace  scenery  with  an  air  of  great  noble- 
ness and  beauty. 

The  complete  series  of  sunset  hues,  from  the  brightest 
light  to  the  deepest  shade,  runs  as  follows  : 

1.  Yellow.  I 3.  Red.  j 6.  Violet-blue. 

2.  Orange.  I 4.  Purple.  i 6.  Grey-blue. 

This,  as  it  were,  normal  series  is  often  interrupted  by 
the  omission  of  one  or  more  of  the  intermediate  hues,  and 
sometimes  begins  as  low  as  the  red  or  even  purple. 


CHAPTER  VT. 


PRODUCTION  OF  COLOUR  BY  FLUORESCENCE  AND 
PHOSPHORESCENCE. 

In  all  the  cases  thus  far  examined,  colour  has  been  pro- 
duced either  by  the  analysis  of  white  light  or  by  subjecting 
it  to  a process  of  subtraction,  as  in  the  examples  mentioned 
in  the  last  chapter.  The  very  astonishing  discovery  of 
Stokes,  however,  has  proved  that  colour  can  be  produced  in 
a new  and  entirely  different  way.  If,  in  a darkened  room 
the  pure  violet  light  of  the  spectrum  be  allowed  to  fall  on 
a plate  or  wineglass  made  of  uranium  glass,  these  articles 
will  not  reflect  violet  light  to  the  eye  as  would  be  expected, 
but  will  glow  with  a bright-green  light,  looking  in  the  dark- 
ness almost  as  though  they  had  suddenly  become  self-lumi- 
nous. This  kind  of  glass  has,  then,  the  extraordinary  prop- 
erty of  entirely  altering  the  colour  of  the  light  that  falls  on 
it,  and  of  causing  the  light  to  assume  a quite  different 
hue.  But,  as  colour  depends  on  wave-length,  we  are  led  to 
ask  whether  this  property  of  the  original  beam  of  light  is 
also  affected  by  the  uranium  glass.  Stokes  proved  conclu- 
sively that  this  is  the  case,  and  that  in  all  such  experiments 
the  length  of  the  wave  is  made  greater.  It  would  appear 
that  the  waves  of  light  act  on  the  atoms  which  make  up  (or 
surround)  the  molecules  of  the  glass,  and  set  them  in  vibra- 
tion ; they  continue  in  vibration  for  some  little  time  after- 
ward, at  a rate  of  their  oicn  selection.,  which  is  always  less 
than  that  of  the  waves  of  light  which  gave  the  first  impulse. 
Being  in  vibration,  they  act  as  luminous  centres,  and  com- 


PRODUCTION  OF  COLOUR  BY  FLUORESCENCE,  ETC.  63 

municate  vibrations  to  the  external  ether,  and  this  is  the 
green  light  that  finally  reaches  the  eye.  The  action  takes 
place  not  only  on  the  surface  of  the  glass,  but  deep  in  its 
interior,  so  that,  if  the  experiment  be  made  with  a thick 
cube  of  the  glass,  it  actually  appears  milky  and  almost 
opaque,  owing  to  the  abundant  flood  of  soft  green  light 
which  it  pours  out  in  all  direptions.  It  is  not  even  neces- 
sary to  employ  as  a source  of  illumination  the  pure  violet  of 
the  spectrum  ; sunlight  streaming  through  blue  cobalt  glass 
answers  as  well,  and  the  sharp  change  from  the  violet-blue 
to  the  milky  "green  is  quite  as  astonishing. 

Under  ordinary  daylight  uranium  glass  scatters  in  all 
directions  a bluish -green  light,  which  is  due  to  the  cause 
above  mentioned,  but  the  light  which  passes  through  its 
substance  is  merely  tinged  yellow.  Both  these  tints  make 
their  appearance  in  daylight,  and  by  their  combination 
communicate  to  articles  made  of  this  glass  a peculiar  and 
rather  beautiful  appearance.  Candle-light  or  gas-light  fur- 
nishes but  a scanty  supply  of  blue  and  violet  rays,  hence 
this  kind  of  illumination  robs  uranium  glass  entirely  of 
its  charm,  and  the  articles  made  of  it  assume  a dull  yel- 
lowish hue  which  is  neither  striking  nor  attractive.  There 
are  many  salts  which  have  this  property  in  a high  degree  : 
among  the  best  known  is  the  platino-cyanide  of  barium, 
which  presents  appearances  similar  to  those  above  men- 
tioned. Thallene,  an  organic  substance  derived  from  coal- 
tar,  and  described  by  Morton,  must  also  be  classed  with 
uranium  glass.*  Drawings  made  with  this  substance  on 
white  paper,  by  daylight  appear  yellowish,  but  when  placed 
under  a violet  or  blue  illumination  flash  into  sudden  bril- 
liancy, and  scatter  in  all  directions  a strong  greenish  light. 
There  are  many  liquids  which  have  the  same  property, 
and  which  display  different  colours  when  acted  on  by  violet 
light,  but  for  an  account  of  them  we  must  refer  the  curi- 


* See  “Chemical  News,”  December,  1872. 


64 


MODERN  CHROMATICS. 


ous  reader  to  Dr.  Pisko’s  work  on  the  liuorescence  of 
light.* 

Before  passing  from  this  subject  it  maybe  as  well  to  add 
that  phosphorescence  also  often  gives  rise  to  colours  which 
more  or  less  closely  resemble  those  of  fluorescence.  If  tubes 
fllled  with  the  sulphides  of  barium,  strontium,  calcium,  etc., 
be  placed  in  a dark  room  and  illuminated  for  an  instant 
by  a beam  of  sunlight,  by  the  electric  light,  or  by  burning 
magnesium  wire,  they  will  display  a charming  set  of  tints 
for  some  minutes  afterward.  Some  will  shine  with  a soft 
violet  light,  others  will  disj^lay  an  orange  or  yellow  glow  ; 
delicate  blues  will  make  their  appearance,  and  will  contrast 
well  with  the  red  hues,  the  latter  resembling  in  the  dark- 
ness living  coals  of  Are.  The  tints,  as  such,  are  very  beau- 
tiful and  suggestive,  though  of  course  no  direct  application 
can  be  made  of  them  to  artistic  purposes. 


* “ Die  Fluorcsccnz  des  Lichtes,”  F.  J.  Fisko,  Vicuna. 


CHAPTER  VII. 


ON  THE  PRODUCTION  OF  COLOUR  BY  ABSORPTION 

The  colours  produced  by  the  dispersion,  interference, 
and  polarization  of  light  have  great  interest  from  a purely 
scientific  point  of  view,  and  are  also  valuable  in  helping  us 
to  frame  a true  theory  of  colour,  but  it  is  to  the  phenomena 
of  absorption  that  the  colours  of  ordinary  objects  are  almost 
entirely  due.  The  pigments  used  by  painters,  the  dyes  em- 
ployed by  manufacturers,  the  colouring-matter  of  flowers, 
trees,  rocks,  and  water,  all  belong  here.  Let  us  begin  our 
study  of  this  subject  with  a fragment  of  stained  glass. 
When  we  place  the  glass  flat  on  a surface  of  black  cloth, 
and  expose  it  to  ordinary  daylight,  we  find  that  it  reflects 
light  to  the  eye  just  as  a piece  of  ordinary  window-glass 
would  under  similar  circumstances,  and  this  light  is  white, 
not  coloured.  In  this  experiment,  the  rays  of  light  which 
reach  the  eye  have  been  reflected  from  the  mere  surface  of 
the  plate  of  glass,  those  rays  which  penetrate  its  interior 
being  finally  absorbed  by  the  black  cloth  underneath,  and 
never  reaching  the  eye  at  all.  If  we  now  raise  the  glass 
and  allow  the  light  of  the  skj^  to  pass  through  it  and  fall 
on  the  eye,  we  find  that  it  has  been  coloured  ruby-red.  The 
light  of  a candle-  or  gas-flame  is  affected  in  the  same  way, 
and  a beam  of  sunlight  streaming  through  the  plate  of  glass 
falls  on  the  opposite  wall  as  an  intensely  red,  luminous 
spot.  Our  first  and  very  natural  impression  is,  that  the 
stained  glass  has  the  power  of  altering  the  quality  of  light — 
that  the  white  light  is  in  some  vay  actually  transmuted 


GG 


MODERN  CHROMATICS. 


into  red  light.  This  seems  to  be  the  universal  impression 
among  those  who  have  not  particularly  examined  the  mat- 
ter. We  saw,  in  Chapter  II.,  that  with  a prism  we  could 
analyze  white  light,  and  sort  out  the  waves  composing  it 
according  to  their  length,  and  that  the  sensation  which  the 
waves  produced  on  the  eye  varied  with  their  length,  the  long- 
est giving  red,  the  shortest  violet.  The  prism  can  also  be  ap- 
plied to  the  study  of  the  matter  now  under  consideration.  A 


Fia.  14.— Red  Glass  placed  over  Slit  In  Black  Cardboard. 


screen  of  black  pasteboard  is  to  be  prepared  with  a narrow 
slit  cut  in  its  centre  ; over  the  slit  a piece  of  stained  glass  is 
to  be  fastened,  as  indicated  in  Fig.  14.  If,  now,  this  arrange- 
ment be  })laced  in  front  of  a window,  matters  can  be  so 
contrived  that  white  light  from  a cloud  shall  fall  upon  the 
slit  and  traverse  the  stained  glass  ; it  will  afterward  reach 


Fio.  15.— Spectrum  of  Light  transmitted  through  Red  Glass. 


the  prism  which  will  analyze  it.  On  making  this  experi- 
ment we  find  that  the  result  is  similar  to  what  is  indicated 
in  Fig.  15  : the  prism  informs  us  that  the  transmitted  beam 


ON  THE  PRODUCTION  OF  COLOUR  BY  ABSORPTION.  67 


consists  mainly  of  red  light  ; a little  orange  light  is  also 
present.  The  experiment  can,  however,  he  made  in  a more 
instructive  way,  by  covering  only  half  of  the  slit  with 
the  plate  of  glass.  On  repeating  it  v/ith.  this  modification, 
we  obtain  side  by  side  an  analysis  of  the  white  light  direct 
from  the  cloud,  and  of  the  light  which  has  traversed  the 
ruby  glass  ; the  result  is  indi6ated  in  Fig.  16,  and  we  see 


Fi&.  16.— Spectrum  of  White  and  Ked  Light  compared. 


at  a glance  that  the  solution  of  the  whole  matter  is  sim- 
ply this  : the  ruby  glass  is  able  to  transmit  the  red  rays, 
but  it  stops  all  the  others  ; these  last  it  absorbs — hence 
we  say  it  produces  its  colour  by  absorption.  The  other  rays 
are  in  fact  converted  into  heat,  and  raise  the  temperature 
of  the  glass  to  a trifling  extent.  The  experiment  can  be 
varied  somewhat  without  affecting  the  result  ; if  a solar 
spectrum  be  projected  on  a screen,  as  described  in  Chapter 
II.,  we  shall  find,  when  we  look  at  it  through  the  ruby  glass, 
that  we  can  see  only  the  red  space,  light  from  the  other  col- 
oured spaces  not  being  able  to  penetrate  the  glass ; and 
finally,  when  we  hold  our  plate  of  glass  directly  in  the  paths 
of  the  coloured  rays,  we  shall  notice  that  it  stops  all  ex- 
cept those  that  are  red.  These  simple  fundamental  experi- 
ments prove  that  the  ruby  glass  does  not  transmute  white 
light  into  red,  but  that  it  arrests  certain  rays,  and  converts 
them  into  a kind  of  force  which  has  no  effect  on  the  eye  ; 
the  rays  which  are  not  arrested  finally  reach  the  eye  and 
produce  the  sensation  of  colour. 


G8 


• MODERN  CHROMATICS. 


For  more  careful  examinations  of  the  coloured  light 
transmitted  by  stained  glass  a spectroscope  with  one  flint- 
glass  prism  can  advantageously  be  used.  The  stained  glass 
is  to  be  fastened  so  that  it  covers  one  half  of  the  slit,  and 
then  we  shall  have,  placed  side  by  side,  the  spectrum  due 
to  the  glass  and  a prismatic  one  for  comparison.  In  this 
latter  the  fixed  lines  will  be  present,  and  we  can  use  them  as 
a kind  of  natural  micrometer  for  mapping  down  our  results. 
There  is,  however,  another  point  to  be  attended  to.  When 
we  come  to  examine  the  red  glass  carefully  with  the  spec- 
troscope, we  find  that  it  not  only  transmits  the  red  rays 
powerfully,  but  that  a little  of  the  orange  rays  also  passes 
through  with  still  smaller  j)ortions  of  the  green  and  blue 
rays.  Hence  we  are  dealing  not  only  with  spaces  in  the 
spectrum,  but  with  the  relative  intensities  of  the  coloured 
light  filling  those  spaces.  Tt  is  diflicult,  or  rather  impos- 


Fio.  17. — Spectmm,  showinfr  the  Extent  and  Intensity  of  the  Coloured  Light  transmitted 
by  Red  Glass.  The  shaded  portion  represents  the  transmitted  light. 


sible,  to  represent  the  different  intensities  by  shading  on 
paper  ; hence  physicists  have  adopted  a certain  convention 
which  removes  this  trouble,  and  enables  them  to  express 
differences  in  luminosity  readily  and  accurately.  All  this 
is  accomplished  by  drawing  a curve,  and  agreeing  that  dis- 
tances measured  upward  to  it  shall  represent  different  de- 
grees of  luminosity.  We  agree,  then,  to  let  the  entire  rec- 
tangle A H O X,  Fig.  IT,  represent  a solar  spectrum,  with 


ON  THE  PRODUCTION  OF  COLOUR  BY  ABSORPTION.  69 

its  different  colours  properly  arranged,  and  having  their 
natural  or  normal  luminosities,  and  in  this  rectangle  we 
draw  the  curve  furnished  hy  the  red  glass  (Fig.  17).  We 
find  that  it  is  highest  in  the  red  space  ; hut  even  here  it 
reaches  only  about  half  way  up,  showing  that  the  luminos- 
ity of  the  transmitted  red  light  is  only^half  as  great  as  that 
of  the  same  light  in  the  spectrum  ; in  the  orange  space  it 
falls  rapidly  off,  the  curve  sinking  with  a steep  slope  ; after 
that  it  runs  out  into  the  green  and  blue,  almost  to  the  vio- 
let, in  such  a way  as  to  indicate  that  the  red  glass  transmits 
minute  quantities  of  these  different  kinds  of  coloured  light. 
The  luminosity,  then,  of  all  the  transmitted  rays,  except 
the  red,  being  quite  feeble,  the  light  which  comes  through 
appears  pure  red.  Making  an  examination  of  an  orange- 
yellow  glass  in  the  same  way,  we  obtain  the  curve  shown  in 
Fig.  18  : this  glass,  it  appears,  transmits  most  of  the  red, 


Fro.  18. — The  shaded  portion  shows  the  amount  of  light  transmitted  by  an  orange- 
coloured  glass. 


orange,  and  yellow  rays,  with  much  of  the  green  and  a 
little  of  the  blue.  Here,  of  course,  the  orange  and  yellow 
rays  after  transmission  make  up  an  orange-yellow  hue,  and 
the  green  and  red  rays  by  their  union  reproduce  the  same 
colour,  as  we  shall  see  in  Chapter  X.  Hence  the  final  colour 
is  orange-yellow,  without  the  least  tint  of  red  or  green. 
Taking  next  a green  glass,  we  obtain  another  curve.  Fig.  19, 
showing  that  much  green  light  is  transmitted,  but  along 


TO 


MODERN  CHROMATICS. 


BED  YEL  GREEM  BLUE  VIOLET 


Fig.  19. — The  shaded  portion  represents  the  amount  of  light  transmitted  by  green  glass. 


witli  it  some  red  and  some  blue.  Blue  gdass  shows  the 
cyan-blue  weakened,  the  ultramarine-blue  and  violet  strong; 
the  green  .is  very  weak,  so  also  are  yellow  and  orange  ; the 
red  is  mostly  absent,  except  a feeble  extreme  red.  The  re- 
sult is  of  course  a violet-blue  (Fig.  20).  A purple  glass  is 


BED  YEL.  GREEN  BLUE  VIOLET 


Fig.  20.— The  shaded  portion  represents  the  amount  of  light  transmitted  by  blue  glass. 


found  to  absorb  the  middle  of  the  spectrum,  i.  e.,  the  yel- 
low, green,  and  cyan-blue  ; the  red  and  violet  are  also  en- 
feebled, but  are  at  all  events  far  stronger  than  the  other 
transmitted  rays.  We  have,  then,  as  a final  result,  red, 
ultramarine-blue,  and  violet,  which  being  mingled  make 
purple.  It  is  evident  from  these  experiments  that  the  col- 
ours produced  by  absorption  are  not  simple,  like  those  fur- 
nished by  the  prism,  but  are  resultant  hues,  produced  by 
the  mixture  of  many  different  kinds  of  coloured  light  hav- 


ON  THE  PKODUCTION  OF  COLOUR  BY  ABSORPTION.  71 

ing  varying  degrees  of  brightness.  On  this  account,  and  by 
reason  of  the  tendency  of  many  kinds  of  stained  glass  to 
absorb  to  a considerable  extent  all  kinds  of  coloured  light 
presented  to  them,  it  happens  that  stained  glass  furnishes 
us  with  coloured  light  inferior  in  purity  and  luminosity  to 
that  obtained  by  the  use  of  a prism.  Nevertheless  these 
colours  are  the  purest  and  most  intense  that  we  meet  with  in 
daily  life,  and  far  surpass  in  brilliancy  and  saturation  those 
produced  by  dyestuffs  or  pigments. 

There  is  one  property  which  probably  all  substances 
possess  which  produce  colour  by  absorption,  upon  which  a 
few  words  must  be  now  bestowed.  If  we  cause  white 
light  to  pass  through  a single  plate  of  yellow  glass,  the 
rays  which  reach  the  eye  will  of  course  be  coloured,  yellow. 
Add  now  a second  plate  of  the  same  glass,  and  the  light 
which  traverses  the  double  plate  assumes  a somewhat  dif- 
ferent appearance  ; it  evidently  is  not  so  luminous,  and  its 
colour  is  no  longer  quite  the  same.  Using  six  or  eight 
plates  of  the  yellow  glass,  we  find  that  the  transmitted 
light  appears  orange.  If  the  same  experiment  be  repeated, 
using  a considerable  number  of  plates  of  the  same  glass, 
the  colour  will  change  to  dark  red.  From  this  it  appears 
that  the  colour  of  the  transmitted  beam  of  light  depends 
somewhat  on  the  thickness  of  the  absorbing  medium.  This 
change  in  the  case  of  some  liquids  is  very  considerable : 
thin  layers,  for  example,  of  a solution  of  chloride  of  chro- 
mium transmit  green  light  mainly,  and  so  imitate  the  ac- 
tion of  a plate  of  green  glass  ; thick  layers  of  the  same 
liquid  transmit  less  light  in  general,  but  the  dominant 
colour  is  red,  and  objects  viewed  through  them  look  as 
they  would,  seen  through  a plate  of  dark-red  glass.  This 
curious  property  is  easily  explained  by  an  examination  of 
the  action  of  the  liquid  on  the  prismatic  spectrum.  In  Fig. 
21  the  curve  represents  the  relative  intensity  of  the  coloured 
light  in  different  portions  of  the  spectrum.  If  we  cut  off 

successively  slices  of  the  rectangle,  as  is  done  in  Figs.  22 
4 


72 


MODERN  CHROMATICS. 


and  23  we  obtain  tlie  curves  corresponding  to  a greater 

'and  greater  tldckness  of  liquid,  and  it  is 

we  shall  have  the  state  of  things  indicated  in  T g.  ^ , 


\ t 

5 L 

< L 

\ 

\ 

/ 

Dl  lie 

V/ni  FT 

RED 


Fia.  2i.-Chloride  of  Cbromium;  Eflect  produced  by  o Thick  Layer. 

curve  is  about  the  same  as  for  red  glass  (Fig.  li),  and  the 
final  colour  is  red.  This  is  an  extreme  case,  but  in  stained 
glasses,  pigments,  dyestuffs,  etc.,  there  is  generally  a ten- 


ABC 


■flGD Tel  green  blue  violet 

Fig.  23.-Chloride  of  Chromium  ; Effect  produced  by  a very  Thick  Layer. 

dency  toward  the  production  of  effects  of  this  kind,  some 
of  which  will  hereafter  be  noticed.  ^ 

The  colours  of  painted  glass  are  similar  to  t lose  o 
stained  glass  in  origin  and  properties  ; both  are  intense 
rather  free  from  admixture  with  white  light,  and  capable 


ON  THE  PRODUCTION  OF  COLOUR  BY  ABSORPTION.  73 

of  a high  degree  of  luminosity.  In  these  respects  they  far 
surpass  the  colours  of  pigments,  which  compared  with  them 
appear  feeble  and  dull,  or  pale.  Owing  to  this  circum- 
stance, chromatic  combinations  may  be  successfully  worked 
out  in  stained  glass,  which  would  prove  failures  if  attempt- 
ed with  pigments  or  dyestuffs.  Hence  also  the  wonderfully 
luminous  appearance  of  paintings  on  glass  viewed  in  a prop- 
erly darkened  room  : they  surpass  in  some  respects  oil  or 
water-colour  paintings  to  such  a degree  that  the  two  are  not 
to  be  mentioned  together.  There  is  no  doubt  but  that 
glass-painting  offers  advantages  for  the  production  of  real- 
istic effects  of  colour  and  light  and  shade,  such  as  the  very 
narrow  scale  of  oil  and  water-colour  utterly  denies  ; and 
yet  great  artists  seem  to  reject  this  process,  and  severely 
confine  themselves  to  work  on  canvas  or  paper,  choosing  to 
depend  for  their  effects  rather  on  pure  technical  skill  and 
artistic  feeling. 

If  we  place  on  a sheet  of  white  paper  a fragment  of  pale- 
blue  glass,  it  will  display  its  colour,  though  not  so  brilliantly 
as  when  held  so  that  the  light  of  the  window  streams  through 
it  directly.  The  reason  is  very  evident : the  light  which 
penetrates  the  glass  falls  on  the  paper  and  is  refiected  by  it 
back  through  the  glass  to  the  eye.  The  light  then  traverses 
the  glass  twice,  but  this  is  not  the  only  cause  of  its  inferior 
luminosity,  for  a double  plate  of  the  same  glass  held  before 
the  window  appears  still  far  brighter  than  the  single  glass 
on  ihe  paper.  The  other  reason  is  that  the  paper  itself  re- 
flects only  a small  amount  of  the  light  falling  on  it.  Upon 
examining  the  matter  more  closely  we  find  also  that  the  blue 
glass  reflects  from  its  surface  quite  a quantity  of  white  light, 
which,  when  mingled  with  the'  coloured  light,  renders  it 
somewhat  pale.  If,  now,  we  grind  up  into  a very  fine  pow- 
der some  of  the  blue  glass,  we  obtain  the  pigment  known 
as  smalt,  and,  after  mixing  it  with  water,  we  can  wash  our 
white  paper  with  a thin  layer  of  it.  When  it  dries  the 


74 


MODERN  CHROMATICS. 


paper  will  be  coloured  blue,  but  the  hue  will  be  neither  so 
luminous  nor  so  intense  as  that  of  the  light  directly  trans- 
mitted by  the  blue  glass  when  held  before  a window.  Its 
origin,  however,  is  similar  : the  white  light  after  traversing 
a layer  of  the  minute  blue  particles  reaches  the  paper,  and 
is  redected  backward  once  more  through  them  toward  the 
eye.  In  this  process  many  coloured  rays  suffer  absorption, 
and  only  a small  portion  of  the  constituents  of  the  original 
l)cam  tinally  reach  the  eye.  In  the  original  experiment, 
where  the  blue  glass  Avas  simply  laid  on  the  white  paper,  it 
sometimes  happened  that  the  white  light  regularly  reflected 
from  its  first  surface  mingled  itself  Avith  the  coloured  light 
ami  caused  it  to  look  paler,  but  it  was  always  possible  to 
arrange  matters  so  that  this  damaging  coincidence  did  not 
occiir?  In  the  exiieriment  witli  the  blue  powder  spread  on 
the  ])aper  this  is  impossible,  for  the  surlaces  of  the  little 
particles  lie  Avith  all  possible  inclinations,  so  that,  hold  the 
paper  as  we  will,  it  is  sure  to  reflect  much  white  along  with 
its  coloured  light.  What  we  have,  then,  to  expect  when 
])igments  in  dry  jiowder  are  spread  on  Avhite  paper  is,  that 
they  Avill  reflect  only  a moderate  quantity  of  coloured  light 
to  the  eye,  and  that  it  will  be  rendered  someAvhat  pale  by 
admixture  Avith  Avhite  light. 

With  the  aid  of  a little  liand  spectroscope  these  points 
are  readily  demonstrated  : Avhen  Ave  direct  the  instrument 
toward  our  blue  paper,  we  find  that  all  the  colours  of  the 
spectruiu  are  iireseiit  in  considerable  quantities  hence  some 
white  liijht  must  be  reflected  from  the  paper  ; we  also  notice 
that  the"  red,  yellow,  orange,  and  green  rays  are  present  in 
less  quantity  than  in  an  ordinary  prismatic  spectrum— hence 
the  curve  for  the  smalt-paper  is  like  that  given  in  Fig.  24. 
In  making  examinations  with  the  spectroscope  of  the  col- 
oured light  reflected  from  painted  surfaces,  it  is  advanta- 
o-cous  to  use  simultaneously,  along  with  the  strip  of  painteff 
paper,  one  which  is  white  and  a third  which  is  black  It 
has  been  found  by  the  author  that  paper  painted  dead-black 


ON  THE  PRODUCTION  OF  COLOUR  BY  ABSORPTION.  75 


with  lampblack,  to  which  has  been  added  just  enough  spirit 
varnish  to  prevent  its  rubbing  off,  but  not  enough  to  cause 
it  in  the  least  degree  to  shine,  reflects  as  much  light  as 
white  paper.  Hence  if  we  set  the  luminosity  of  white  paper 
as  100,  that  of  dead-black  paper  will  be  5.  Now,  when  a 


ABC  D E F G H 


Fig.  24. — Curve  for  Smalt-paper  : the  shaded  portion  represents  the  light  reflected 
hy  smalt-paper. 


strip  of  this  black  paper  is  placed  before  the  slit  of  the  spec- 
troscope it  acts  like  white  paper  seen  under  a feeble  illumi- 
nation, and  consequently  furnishes  a complete  though  not  a 
very  luminous  spectrum.  By  using,  then,  a black-and-white 
strip  along  with  the  one  which  has  been  painted,  we  can 
ascertain  several  facts  which  may  best  be  explained  with 
the  help  of  an  example.  Let  us  first  select  vermilion  in  dry 
powder,  and  undertake  an  examination  of  its  optical  proper- 
ties in  this  way.  We  find  that  the>red  of  its  spectrum  is 
about  as  powerful  as  the  red  in  the  spectrum  from  white 
paper,  and  that  the  other  colours,  though  all  present,  are 
not  much  if  any  stronger  than  those  from  the  black  paper. 
This  is  all  we  can  demand  from  any  pigment  : it  reflects  to 
the  eye  its  full  share  of  the  rays  it  professes  to  reflect,  and 
they  are  not  mingled  with  more  white  light  than  is  reflected 
by  dead-black  paper.  Emerald-green  when  tested  in  this 
way  proves  sensibly  inferior  to  vermilion  : examined  in 
dry  powder  the  green  space  was  bright,  but  less  bright  than 
that  from  white  paper  ; the  other  colours  had  about  the 
same  degree  of  luminosity  as  those  from  the  black  paper. 


76 


MODERN  CHROMATICS. 


except  the  violet,  which  was  not  present.  Chrome-yellow 
reflected  the  red,  orange,  yellow,  and  green  rays  about  as 
brilliantly  as  white  paper  ; the  cyan-blue,  ultramarine,  and 
violet,  about  like  black  paper.  Hence  the  great  luminosity 
of  this  pigment,  for  it  reflects  not  only  the  yellow  rays 
abundantly,  but  also  all  the  other  rays  of  the  spectrum 
which  are  distinguished  for  luminosity.  As  before  re- 
marked, the  sum  of  these  rays  makes  up  yellow.  It  is  plain 
from  these  experiments  that  a painted  surface  can  never  be 
as  luminous  as  one  which  is  white  ; the  most  that  can  be 
demanded  from  a painted  surface  is,  that  it  should  reflect 
its  peculiar  coloured  light  as  powerfully  as  a white  surface 
does  ; the  very  cause  of  its  furnishing  coloured  light  is,  that 
it  fails  to  reflect  all  the  coloured  rays  equally  well.  Hence 
coloured  surfaces  are  always  darker  than  those  which  are 
white.  If  we  set  the  luminosity  of  white  paper  as  100,  that 
of  vermilion  will  be  about  25,  emerald-green  48,  and  chrome- 
yellow  as  high  as  75  or  80. 

These  experiments  can  now  be  repeated  with  the  same 
pigments  covered  by  a layer  of  water.  The  surface  of  the 
water  being  quite  flat,  the  spectroscope  can  be  held  in  such 
a way  as  to  avoid  the  light  directly  reflected  from  the  water, 
and  it  then  becomes  possible  to  observe  certain  changes 
which  the  presence  of  the  water  brings  about.  In  the  case 
of  vermilion  we  lind  that  the  blue  and  violet  portions  of  the 
spectrum  almost  entirely  vanish,  a little  of  the  yellow, 
orange,  and  green  spaces  remains,  and  the  red  is  nearly  as 
powerful  as  before.  This  proves  that  the  presence  of  the 
water  has  greatly  diminished  the  amount  of  xnhite  light  re- 
flected from  the  surfaces  of  the  particles  of  pigment,  but 
has  not  much  affected  the  brilliancy  of  the  reflected  col- 
oured light.  Experiments  with  emerald-green  and  chrome- 
yellow  give  corresponding  results  ; less  light  in  general  is 
reflected,  but  it  is  somewhat  purer,  there  being  not  so  much 
white  light  mingled  with  it.  By  immersing  our  pigments 
in  oil  or  varnish  we  push  these  effects  still  further  : the 


ON  THE  PRODUCTION  OP  COLOUR  BY  ABSORPTION.  77 

pigments  appear  darker,  but  the  colour  is  richer,  and  more 
nearly  .free  from  white  light.  The  explanation  of  these 
changes  is  well  known  to  physicists  : they  depend  upon  the 
fact  that  light  moving  in  a rare  medium  like  the  air  is  abun- 
dantly reflected  when  it  strikes  on  a dense  substance  like  a 
pigment ; but  if  the  pigment  be  placed  under  water  we 
have  then  light  moving  in  a dense  medium  (water),  and 
striking  on  one  which  is  only  a little  more  dense  (pigment)  : 
hence  but  little  white  light  will  be  reflected  from  the  sur- 
face of  the  small  particles.  The  coloured  light  which  is  so 
abundantly  furnished  by  the  pigment,  even  under  water, 
has  its  source  in  reflections  which  take  place  in  the  interior 
of  the  somewhat  coarsely  grained  particles  of  the  pigment 
itself.  If  the  pigment  is  naturally  fine-grained,  and  also  is 
mixed  with  a liquid  like  oil,  having  about  the  same  optical 
density  as  itself,  scarcely  any  light  will  be  reflected  from  it, 
coloured  or  otherwise.  Prussian-blue  and  crimson-lake, 
ground  in  oil,  are  good  examples.  In  order  to  exhibit  their 
colours  it  is  necessary  either  to  spread  them  in  thin  layers 
over  a light  surface,  or  to  mix  them  with  a white  pigment ; 
alone  by  themselves  they  appear  very  dark,  the  Prussian- 
blue,  indeed,  almost  black.  Many  other  pigments  are  more 
or  less  affected  in  the  same  way  by  the  presence  of  oil  or 
varnish. 

From  what  has  been  stated  above  it  follows  that  the 
medium  with  which  pigments  are  mixed  has  an  important 
influence  on  their  appearance.  In  drawings  executed  in 
coloured  chalks,  and  in  oil-paintings,  we  have  the  two  ex- 
tremes, works  in  water-colour  being  intermediate.  Hence 
oil-painting  is  characterized  by  the  richness  of  the  colouring 
and  the  transparency  and  depth  of  its  shadows,  while  in 
pastel  drawings  the  tints  are  paler,  the  shadows  less  intense, 
and  over  the  whole  is  spread  a soft  haze  which  lends  itself 
readily  to  the  accurate  imitation  of  skies  and  distances. 
Changes  in  the  medium  are  sometimes  a source  of  embar- 
rassment to  the  painter.  This  is  particularly  true  in  the 


78 


MODERN  CHROMATICS. 


process  of  fresco-painting,  and  also  to  some  extent  in  that 
of  water-colour  : as  long  as  the  pigment  is  moist  it  appears 
darker  than  afterward  when  dry,  and  it  is  necessary  for  the 
artist  in  laying  on  each  wash  to  make  a proper  allowance 
for  these  changes  ; this  is  one  of  the  minor  causes  that  ren- 
der the  process  of  painting  in  water-colours  more  difficult 
than  that  in  oils. 

As  has  already  been  stated,  when  we  obtain  our  coloured 
light  from  pigments,  it  is  apt  to  be  more  mingled  with  white 
light  than  when  stained  glass  is  used  ; but,  besides  this,  it 
is  inferior  to  that  from  stained  glass  in  the  matter  of  lumi- 
nosity. Tlie  range  of  illumination  in  our  houses  is  small, 
so  that  practically  the  scale  of  light  at  the  disposal  of  the 
painter  in  oils  or  water-colours  is  quite  limited  ; in  point  of 
fact  he  is  obliged  by  the  necessities  of  the  case  to  employ 
means  which  are  quite  inadequate  : hence  the  extraordinary 
care  with  which  he  husbands  his  resources  in  the  matter  of 
light  and  shade,  and  his  constant  struggle  for  excellence 
and  decision  in  colouring.  Muddy  and  dirty  colours  are 
instantly  recognized  to  be  such  under  a feeble  illumination, 
even  though  they  have  passed  muster  under  the  blaze  of 
full  sunlight.  Almost  any  surface  looks  beautiful  if  very 
brightly  illuminated  ; the  eye  is  dazzled,  and  remains  un- 
conscious of  defects  that  are  instantly  exposed  under  the 
feebler  light  of  a gallery. 

The  colours  which  are  exhibited  by  woven  fabrics  are 
due,  like  those  of  stained  glass,  to  absorption.  In  the  case 
of  silk  and  avooI  the  dye  penetrates  tlie  tibres  through  and 
through,  so  that  under  the  microscope  they  ha\*e  much  the 
same  appearance  as  fine  threads  of  stained  glass.  When 
AA'hite  light  falls  upon  a bundle  of  such  coloured  fibres,  a 
portion  is  reflected  uncoloured  from  the  surface  of  the  top- 
most fibres,  while  another  portion  penetrates  to  the  rear 
surfaces  of  these  same  fibres  and  there  is  again  subdivided, 
some  rays  penetrating  still  deeper  into  the  bundle,  while 
others  returning  to  the  upper  surface  emerge  coloured. 


ON  THE  PRODUCTION  OF  COLOUR  BY  ABSORPTION.  79 

This  process  is  repeated  on  each  deeper-lying  set  of  fibres, 
and  the  result  is  that  a good  deal  of  strongly  coloured  light 
is  sent  to  the  eye,  mingled  with  a portion  due  to  the  surface 
layers,  which  is  more  faintly  coloured  ; there  is  in  addition 
a small  portion  which  is  quite  white.  It  will  be  seen  that 
the  reflective  power  of  the  fibres  is  an  important  element  in 
this  process,  for  all  the  colou]:ed  light  which  reaches  the  eye 
is  sent  there  by  reflection.  If  we  take  similar  structures  of 
silk  and  wool,  we  can  compare  directly  the  lustre  or  reflective 
power  of  the  individual  fibres,  with  the  aid  of  a lens  mag- 
nifying ten  or  fifteen  diameters.  A silk-cocoon  and  a piece 
of  white  felting  answer  very  well  for  this  purpose,  and 
when  they  are  compared  under  the  microscope  it  is  very 
evident  that  the  natural  lustre  of  the  silk  is  greatly  superior 
to  that  of  the  wool.  On  comparing  in  this  way  the  felting 
with  white  cotton  batting,  it  will  be  found  that  the  wool 
surpasses  the  cotton  in  lustre,  the  latter  appearing  almost 
dead-white  and  free  from  sparkle.  It  follows  from  this  that 
the  coloured  light  which  is  reflected  from  silk  is  more  satu- 
rated or  intense,  and  appears  richer,  than  that  from  wool. 
The  fibres  of  silk  also  can  be  made  to  lie  in  straight,  paral- 
lel, compact  bundles,  which  enables  them  to  reflect  the  white 
light  in  definite  directions,  whereas  woollen  fabrics  reflect  it 
equally  well  in  all  directions.  It  results  from  this  that  a 
fabric  of  silk  is  capable,  according  to  circumstances,  of  ex- 
hibiting a rich  saturated  colour  nearly  free  from  white 
light,  or  it  may  reflect  much  white  light  and  exhibit  a pale 
colour.  This  sparkling  play  of  colour  is  beautiful,  and 
causes  the  more  uniform  appearance  shown  by  woollen 
fabrics  to  appear  dull  and  tame.  On  the  other  hand,  the 
superior  transparency  of  the  dyed  fibres  of  wool  over  those 
of  cotton  give  to  the  colours  of  the  former  material  a cer- 
tain appearance  of  richness  and  saturation,  and  cause  the 
tints  of  the  cotton  to  appear  somewhat  opaque. 

In  velvet  the  attempt  is  made  to  suppress  all  surface-light, 
and  to  display  only  those  rays  which  have  penetrated  deeply 


80 


MODERN  CHROMATICS. 


amoiKv  the  fibres,  and  have  become  highly  coloured.  This  is 
accomplished  by  presenting  to  the  light  a surface  M'hich  is 
entirely  composed  of  the  ends  of  fibres,  and  consequently 
which  has  little  or  no  capacity  for  reflecting  light.  Ihe 
rays  then  penetrate  between  the  fibres  thus  set  up  on  end, 
and,  after  wandering  among  them,  finally  again  in  some 
small  quantity  reach  the  surface  as  richly  coloured  light, 
which  produces  its  full  effect  iindimimshcd  by  any  admix- 
ture of  white  light  from  the  surface.  In  the  case  of  silk- 
velvet  the  desired  effect  is  for  the  most  part  actually  real- 
ized : the  colours  are  rich,  and  an  examination  with  a lens 
shows  that  scarcely  any  of  the  fibres  reflect  white  light 
even  when  the  fabric  is  held  in  unfavouralile  positions  It 
cotton-velvet  is  subjected  to  a similar  examiiiation  under  a 
lens  it  will  be  found  to  reflect  much  surface-light,  particu- 
larly when  not  quite  new,  and  the  surface  will  present  a 
broken,  rough  appearance,  quite  dilTerent  from  that  of  its 

more  aristocratic  rival.  „ m i ^ 

It  would  appear  that  at  present  it  is  actually  possible  to 
employ  for  woven  fabrics  dyes  which  furnish  coloured  light 
havino-  a degree  of  intensity  and  purity  whicli  is  actually 
iiiidcsTrablo.  This  is  the  case  with  some  of  the  aniline  dyes. 
Dresses  dyed  with  some  of  them,  when  seen  in  full  daj  light, 
act  on  the  eye  so  powerfully  that  mere  momentary  inspec- 
tion (rives  rise  to  the  phenomenon  of  accidental  colours  (see 
Chapter  VIll'.).  These  harsh  effects  are  interesting  as  con- 
veviim  certain  information  that  our  dyers  have  already 
touched,  and  indeed  gone  beyond,  the  greatest  allowable 
limits  in  the  matter  of  the  intensity  and  purity  of  then- 
hues  At  least  this  applies  to  large  surfaces,  such  as  com- 
plete dresses,  etc.  In  the  case  of  smaller  articles  such  as 
ribbons,  etc.,  these  intense  colours  are  more  allowable,  ]ust 
as  the  flash  of  diamonds  is  more  tolerable  on  account  of 

their  insiojnificant  size.  . 

We  hWe  seen,  thus  far,  that  the  colours  of  pigments 
and  dyestuffs  are  due  to  absorption,  and  to  this  same  cause 


ON  THE  PRODUCTION  OF  COLOUR  BY  ABSORPTION.  81 


we  must  attribute  the  colours  of  most  objects  wbicb  occur 
in  landscapes.  Two  of  these  are  so  important  that  it  will 
be  worth  while  to  devote  a few  moments  to  their  separate 
consideration  : we  refer  to  the  colour  of  water,  and  to  that 
of  vegetation.  The  colour  of  large  masses  of  water,  such 
as  lakes  and  rivers,  is  so  much  influenced  by  that  of  the 
sky  that  many  persons  consider  it  to  depend  wholly  on  it, 
and  are  disposed  to  doubt  whether  water  has  any  proper 
colour  of  its  own.  It  is  quite  true  that  a small  quantity  of 
pure  water,  such  as  is  contained  in  a drinking-glass,  appears 
perfectly  colourless,  and  that  the  light  from  white  objects 
passes  through  it  without  suffering  sensible  absorption.  If, 
however,  we  allow  the  white  light  from  a porcelain  plate  to 
traverse  a layer  of  pure  distilled  water  two  metres  in  thick- 
ness, it  will  be  found  to  be  tinged  bluish.  This  experiment, 
which  was  first  made  by  Bunsen,  proves  that  an  absorption 
takes  place  along  the  red  end  of  the  spectrum,  and  that 
water  is  really  coloured  in  the  same  sense  as  a weak  solution 
of  indigo.  The  water  of  the  lake  of  Geneva  is  quite  pure, 
being  produced  mainly  by  the  melting  of  glaciers  ; the 
granite  meal  mingled  with  the  water,  being  coarse,  soon 
settles  to  the  bottom,  and  leaves  it  free  from  turbidity. 
Hence  along  the  wonderful  shores  of  this  lake  it  is  easy 
to  repeat  the  experiment  of  Bunsen,  and  to  study  the  colour 
of  this  liquid.  White  objects,  resting  on  the  bottom  in  the 
shallow  places  where  the  depth  is  six  or  eight  feet,  show 
very  plainly  a greenish-blue  hue,  and  the  tint  can  be  exam- 
ined at  different  depths  by  lowering  a piece  of  white  porce- 
lain with  a string.  Even  on  cloudy  days,  when  the  sky  is 
overcast  and  grey,  the  lake  itself  displays  a wonderfully  in- 
tense cyan-blue  colour,  while  on  clear  days,  on  looking 
down  into  its  waters,  one  is  tempted  to  believe  that  it 
is  a vast  natural  dyeing-vat.  When  vegetable  matter  is 
present  in  small  quantity  the  colour  of  water  changes  to  a 
bluish-green  ; many  excellent  examples  occur  among  the 
beautiful  lakes  of  the  Tyrol.  Decaying  organic  matter, 


82 


MODERN  CHROMATICS. 


on  the  other  hand,  tinges  water  brownish,  and  lakes  or 
rivers  of  this  colour  are  apt  to  assume  on  cloudy  days  a 
silver-grey  appearance,  while  under  a clear  sky  they  often 
appear  very  decidedly  blue.  There  seems  to  be  some  reason 
to  believe  that  the  absorptive  action  of  pure  water  on  white 
light  changes  with  its  temperature,  and  that  warm  water  is 
actually  more  deeply  coloured  than  cold  water.  Heat  has 
an  action  of  this  kind  uj)on  many  coloured  substances,  and 
AVild  with  his  photometer  actually  found  that  both  distilled 
and  pump  water  showed  somewhat  stronger  colours  on 
being  heated.  He  accounts  in  this  way  for  the  more  in- 
tense colour  which  it  is  claimed  mountain  lakes  display 
during  the  summer  months. 

The  green  colour  of  vegetation  offers  a rather  peculiar 
case.  AVhen  we  examine  with  the  spectroscope  any  ordi- 
nary green  pigment,  we  find  that  the  red  is  absent  and  the 
blue  and  violet  much  weakened,  as  was  the  case  with  em- 


Fig.  25.— The  shaded  portion  represents  the  light  reflected  by  green  leaves. 


erald-green.  Green  leaves,  however,  furnish  a spectrum  of 
a different  character  : the  extreme  red  is  present ; then 
occurs  a deficiency  of  coloured  light,  which  is  followed  by 
an  orange-red  space  ; next  comes  the  orange,  then  the  yel- 
low, greenish-yellow,  and  yellowish-green  ; after  this  fol- 
lows a little  full  green  ; the  rest  of  the  spectrum  decreases 
rapidly  in  luminosity.  Fig.  25  represents  this  spectrum. 
The  sum  of  all  these  colours  is  a somewhat  yellowish  green, 
which  is  accordingly  the  colour  presented  by  green  leaves 


ON  THE  PRODUCTION  OP  COLOUR  BY  ABSORPTION.  83 

in  white  light.  It  will  he  shown  in  Chapter  X.  that  a mix- 
ture of  red  and  green  light  furnishes  yellow  light,  which 
explains  the  production  of  a yellowish-green  in  this  some- 
what singular  way.  It  follows,  from  the  analysis  just  given, 
that  green  leaves  are  capable  of  reflecting  a considerable 
quantity  of  red  light,  where  surfaces  painted  with  green 
pigments  would  not  have  this  power,  and  consequently 
would  appear  black  or  grey.  Hence  under  the  red  light  of 
the  setting  sun  foliage  may  assume  a red  or  orange-red  hue. 
Corresponding  to  this,  when  the  illumination  is  of  an  orange 
colour,  foliage  will  partake  more  of  this  hue  than  would  be 
the  case  with  ordinary  green  pigments.  Connected  with 
this  is  also  the  great  change  of  colour  which  foliage  expe- 
riences according  as  it  is  illuminated  by  direct  sunlight  or 
by  light  from  the  blue  sky,  the  tint  in  extreme  cases  varying 
from  a yellow  or  slightly  greenish  yellow  up  to  a bluish-green. 

Simler  has  constructed  a simple  and  beautiful  piece 
of  apparatus,  based  on  the  singular  property  which  living 
leaves  have  of  reflecting  abundantly  the  extreme  red  rays 
of  the  spectrum  ; it  is  called  an  erythro scope.  A plate  of 
blue  glass,  stained  with  cobalt,  is  to  be  procured,  having  a 
thickness  such  that  it  will  allow  the  extreme  red  of  the 
spectrum  to  pass,  but  no  orange  or  yellow  ; it  should  also 
transmit  the  small  band  of  greenish-yellow  just  before  the 
fixed  line  E,  and  all  the  green  from  h to  F,  also  all  the  blue 
and  violet.  A plate  of  rather  deeply  coloured  yellow  glass 
is  also  needed  ; this  should  be  capable  of  transmitting  all 
the  light  of  the  spectrum  from  the  farthest  red  up  to  G ; 
that  is  to  say,  it  should  cut  oflc  the  violet  and  blue-violet 
only.  When  a sunny  landscape  ds  viewed  through  these 
two  glasses,  it  assumes  a most  wonderful  appearance  : all 
green  trees  and  plants  shine  with  a coral-red  colour,  as 
though  they  were  self-luminous  ; the  sky  is  cyan-blue,*  the 
clouds  purplish-violet  ; the  earth  and  rocks  assume  various 


* Cyan-blue  is  a greenish-blue. 


84 


MODERN  CHROMATICS. 


tints  of  violet-grey.  Pine-trees  appear  of  a dark-red  hue  ; 
orange  or  yellow  dowers  become  red  or  blood-red  ; greens, 
other  than  those  of  the  foliage,  are  seen  in  their  natural 
tints,  or  at  least  only  a little  more  bluish  ; lakes  preserve 
their  fine  blue-green  colouring,  and  the  play  of  light  and 
shade  over  the  landscape  is  left  undisturbed  ; the  whole 
effect  is  as  though  a magician’s  wand  had  j)assed  over  the 
scene,  and  transformed  it  into  an  enchanted  garden.  For 
the  full  realization  of  these  effects  it  is  essential  that  stray 
light  should  be  j)revented  from  reaching  the  eyes,  and  ac- 
cordingly the  glasses  should  be  mounted  in  an  arrangement 
of  wood  or  pasteboard  which  adaj)ts  itself  to  the  contours 
of  the  face,  and  excludes  as  much  as  possible  diffuse  light. 
On  comparing  the  spectrum  given  by  the  blue  and  yellow 
glasses  with  that  of  green  leaves,  it  will  be  found  that  the 
two  glasses  cut  off  almost  all  the  green  light  furnished  by 
the  leaves,  but  allow  those  green  rays  of  light  to  pass  which 
the  leaves  are  incapable  of  supplying. 

The  colours  which  metals  such  as  copper,  brass,  or  gold 
display,  are  due  to  absorption.  A quantity  of  white  light 
is  reflected  from  the  real  surface,  but  along  with  it  is  min- 
gled a certain  amount  which  has  penetrated  a little  distance 
into  the  substance  of  the  metal,  and  there  has  undergone 
reflection  ; this  last  portion  is  coloured.  If  we  cause  this 
mixture  of  white  and  coloured  light  to  strike  repeatedly  on 
a metallic  surface — for  example,  such  as  gold — we  constant- 
ly increase  the  proportion  of  light  which  has  penetrated 
under  the  surface,  and  has  become  coloured.  A process  of 
this  kind  takes  place  in  the  interior  of  a golden  goblet  ; 
hence  the  colour  in  the  inside  is  deeper  and  more  saturated 
than  on  the  outside.  Some  metals,  like  silver  or  steel,  hard- 
ly show  much  colour  till  the  light  has  been  made  to  strike 
repeatedly  on  their  surfaces  ; when  this  is  done  with  silver, 
the  light  gradually  assumes  a yellow  colour,  while  with 
steel  it  becomes  blue. 


ON  THE  PRODUCTION  OF  COLOUR  BY  ABSORPTION.  85 

The  true  colour  of  metals  must  not  be  confounded  with 
that  which  is  often  given  to  them  by  the  presence  of  a sur- 
face-film of  oxide  or  sulphide  ; such  films  cause  for  the 
most  part  a bluish  appearance,  though  all  the  colours  of  the 
spectrum  may  be  produced  on  metals  in  this  way.  In  fact, 
the  hue  in  these  cases  is  due  to  an  interference  of  light 
caused  by  the  thin  layer  of  oxide,  and  is  quite  distinct  from 
the  actual  colour  of  the  metah  (See  Chapter  IV.) 

Metals,  whether  coloured  or  white,  are  chiefly  remark- 
able for  the  large  quantities  of  light  which  they  are  capable 
of  reflecting.  Measurements  made  by  Lambert  have  shown 
that  the  total  amount  of  light  reflected  by  white  paper  is 
about  forty  per  cent,  of  the  light  falling  on  it.  Silver, 
however,  is  capable  of  reflecting  ninety-two  per  cent. ; steel 
sixty  per  cent.,  etc. 

Polished  surfaces,  particularly  of  metals,  have  another 
property  which  adds  to  their  apparent  brilliancy,  and  in- 
creases their  lustrous  appearance.  Those  portions  of  the 
surface  which  are  turned  away  from  the  light  often  reflect 
but  little,  and  look  almost  black.  This  sharp  contrast  en- 
hances their  brilliant,  sparkling  appeai  ance,  and  raises  them 
quite  above  the  rank  of  surfaces  coloured  by  pigments.  In 
consequence  of  this,  metals  cannot  be  used  along  with  pig- 
ments in  serious  or  realistic  painting  ; they  are  quite  out  of 
harmony,  and  produce  the  impression  that  the  painter  has 
sought  to  help  himself  by  a cheap  trick  rather  than  by  em- 
ploying the  true  resources  of  art.  In  those  cases  ^where 
gold  was  so  extensively  used  during  the  middle  ages  for 
the  backgrounds  of  pictures  of  holy  personages,  or  even  for 
the  adornment  of  their  garments,  the  object  was  far  more 
to  produce  symbolic  than  realistic  representations,  and  here 
the  presence  of  the  gold  was  actually  a help,  as  tending  to 
convey  the  idea  that  the  painting  was  not  the  portrait  of  an 
ordinary  mortal,  but  rather  a childlike  attempt  to  depict 
and  lavishly  adorn  the  ideal  image  of  a venerated  and 
saintly  character.  On  the  other  hand,  this  brilliancy  of 


86 


MODERN  CHROMATICS. 


gold,  with  its  rich  colour,  preeminently  adapts  it  for  the 
purpose  of  inclosing  paintings  and  isolating  them  from  sur- 
rounding objects.  A painted  frame  or  wooden  frame,  inas- 
much as  its  colour  belongs  to  the  same  order  as  those  con- 
tained in  the  picture,  becomes  as  it  were  an  extension  of  it, 
and  is  apt  to  injure  the  harmony  of  its  colouring  ; and,  be- 
sides this,  its  power  of  isolation  is  inferior  to  that  of  gold, 
on  account  of  its  greater  resemblance  to  ordinary  surround- 
ing objects. 


Having  now  considered  with  some  detail  the  colours 
that  are  produced  by  absorption,  it  may  be  well  to  add  a 
few  words  concerning  the  attempts  that  have  been  made  to 
reproduce  colour  by  the  aid  of  photography.  Photographs 
render  accurately  the  light  and  shade,  why  should  they  not 
also  record  the  colours,  of  natural  objects  ? In  1848  E. 
Becquerel  announced  that  he  had  been  able  to  photograph 
the  colours  of  a prismatic  spectrum  falling  on  a silver  plate 
which  had  been  treated  with  chlorine.  These  colors  were 
quite  fugitive,  lasting  only  a few  minutes.  In  1850  Ki^pce 
de  Saint-Victor  and  in  1852  J.  Campbell  claimed  that  they 
had  rendered  these  colours  more  permanent.  In  1862  the 
former  experimenter,  by  washing  the  finished  plates  with  a 
solution  of  dextrin  containing  chloride  of  lead,  obtained 
coloured  pictures  that  lasted  twelve  hours.  In  the  following 
year  lie  still  further  improved  his  process,  the  colours  last- 
ing three  or  four  days  in  rather  strong  daylight.  An  ex- 
amination of  the  details  of  these  memoirs  and  of  the  pic- 
tures indicates  that  the  colours  thus  obtained  are  due  to  a 
greater  or  less  reduction  of  the  film  of  chloride  of  silver, 
and  are,  in  fact,  produced  merely  by  the  interference  of 
light,  and  consequently  have  no  necessary  connection  with 
the  hues  of  the  natural  objects  to  which  they  seem  to  owe 
their  origin.  Hence  we  must  regard  this  problem  as  un- 
solved, and  in  the  present  state  of  our  knowledge  there 


ON  THE  PKODUCTION  OF  COLOUR  BY  ABSORPTION.  87 

does  not  seem  to  be  any  reason  to  suppose  that  it  ever  will 
be  solved.  Why  should  the  red  rays  when  acting  on  a cer- 
tain substance  produce  a red  compound,  the  green  and  vio- 
let rays  green  and  violet  compounds,  and  so  on  with  all  the 
other  coloured  rays  ? But  photography  in  colour  implies 
exactly  this. 

This  problem  has  more  recently  been  handled  in  an  en- 
tirely different  manner,  and  with  a more  hopeful  result, 
from  a practical  point  of  view.  Suppose  we  place  a red 
glass  before  a photographic  camera,  and  photograph  some 
object  with  brilliant  colours — a carpet,  for  example.  We 
shall  obtain  an  ordinary  negative  picture,  which  will  be  en- 
tirely due  to  the  red  light  sent  by  the  carpet  toward  the 
instrument.  Portions  of  the  carpet  having  a different  col- 
our will  not  be  photographed  at  all.  Next  let  us  hold  be- 
fore the  camera  a glass  which  transmits  only  the  yellow 
rays  (if  such  glass  could  be  found),  and  we  shall  obtain  a 
picture  of  the  yellow  constituents  of  the  carpet  ; the  same 
is  to  be  done  with  a blue  glass.  From  these  three  ordinary 
negatives,  three  positive  pictures  are  to  be  made  in  gelatine, 
the  first  being  colored  with  a transparent  red  pigment,  the 
second  with  a yellow,  the  third  with  a blue  pigment.  The 
first  sheet  of  gelatin  will  contain  a red  picture,  due  to  the 
red  parts  of  the  carpet ; the  second  and  third,  similar  yel- 
low and  blue  pictures.  When  these  transparent  coloured 
sheets  are  laid  over  each  other,  we  shall  have  a picture  cor- 
rect in  drawing,  which  will  roughly  reproduce  the  colours 
of  the  carpet.  This  gives  an  idea  of  the  plan  proposed  in 
1869  by  C.  Cross  and  Ducos  du  Hauron,  for  the  indirect 
reproduction  of  colour  by  photography.  In  actual  practice 
the  negatives  were  taken  with  glasses  coloured  orange, 
green,  and  violet  ; these  negatives  were  then  made  to  yield 
blue,  red,  and  yellow  positive  pictures.  This-  process  has 
been  greatly  improved  by  Albert,  of  Munich,  and  by  Bier- 
stadt,  of  New  York.  In  the  final  picture  the  gelatine  is  dis- 
pensed with,  films  of  colour,  laid  on  by  lithographic  stones, 


88 


MODERN  CHROMATICS. 


being  substituted.  The  selection  of  tbe  pigments  is  neces- 
sarily left  to  the  judgment  of  the  operator,  and  in  its  pres- 
ent state  the  process  is  better  capable  of  dealing  with  the 
decided  colours  of  designs  made  by  the  decorator  than  with 
the  pale,  evanescent  tints  of  ISTature. 


APPENDIX  TO  CHAPTER  VII. 

We  give  below  a list  of  pigioents  which,  according  to  Field  and 
Linton,  are  not  affected  by  the  prolonged  action  of  light,  or  by  foul 
air : 


White. 

Orange. 

Zinc-white. 

Orange  vermilion. 

True  pearl-white. 

Jaune  de  Mars. 

Baryta-white. 

Orange  ochre. 

Tin-white. 

Burnt  Sienna. 

Burnt  Roman  ochre. 

lied. 

Vermilion. 
Indian  red. 
Venetian  red. 
Light  red. 

Green. 

Oxide  of  chromium. 
Rinman’s  green. 
Terre-verte. 

Red  ochre. 

Blue. 

Yellow. 

Cadmium-yellow. 

ritramarine. 
Blue  ochre. 

Lemon-yellow. 

Violet. 

Strontia-yellow. 

Purple  ochre. 

Yellow  ochre. 

Violet  de  Mars. 

Raw  Sienna. 
Oxford  ochre. 

Brown. 

Roman  ochre. 

Rubens’s  brown. 

Stone  ochre. 

Vandyck  brown. 

Brown  ochre. 

Raw  umber. 
Burnt  umber. 

Black. 

Cassel  earth. 

Ivory-black. 

Cologne  earth. 

Lampblack. 

Bistre. 

Indian  ink. 

Sepia. 

Graphite. 

Asphalt. 

APPENDIX  TO  CHAPTER  VII. 


89 


White  lead,  smalt  and  cobalt-hlne  are  not  affected  by  light,  but 
are  by  foul  air.  The  last  two  are  considered  permanent  in  water- 
colour painting. 

According  to  Field,  the  tints  of  the  following  pigments  are  not 
affected  by  mixture  with  lime,  consequently  they  are  adapted  for 
use  in  fresco-painting : 


White.  ^ Orange. 


Baryta. 
Pearl. 
Gypsum. 
Pure  earths. 

Red. 

Orange  vermilion. 
Chrome-orange. 
Orange  ochre. 
Jaune  de  Mars. 
Burnt  Sienna. 

Green. 

Vermilion. 

Terre  verte. 

Red  lead. 

Red  ochre. 
Light  red. 
Venetian  red. 
Indian  red. 

Emerald  green. 
Mountain  green. 
Cobalt-green. 
Chrome-green. 

Madder  red. 

Blue. 

Ultramarine. 

Yellovj. 

Smalt. 

Indian  yellow. 

Cobalt. 

Yellow  ochre. 

Purple. 

Oxford  ochre. 
Roman  ochre. 
Stone  ochre. 

Madder  purple. 
Purple  ochre. 

Brown  ochre. 

Brown. 

Raw  Sienna. 

Vandyck  brown. 

Naples  yellow. 

Rubens’s  brown. 

Black. 

Raw  umber. 
Burnt  umber. 

Ivory-black. 

Cassel  earth. 

Lampblack, 

Cologne  earth. 

Black  chalk. 

Antwerp  brown. 

Graphite. 

Bistre. 

As  the  effect  of  light  on  pigments  is  a matter  of  considerable 
importance  to  artists,  particularly  to  those  working  with  the  thin 
washes  used  in  water-colour  painting,  a careful  experiment  on  this 


90 


MODERN  CHROMATICS. 


matter  was  made  by  the  present  writer.  The  washes  laid  on  or- 
dinary drawing-paper  were  exposed  during  the  summer  to  sunlight 
for  more  than  three  months  and  a half,  and  the  effects  noted ; these 
were  as  follows : 


Water-colour  Pigments  that  are  not  affected  by  Light: 


Red. 

Indian  red. 
Light  red. 

Green. 
Terre  verte. 


Orange. 

Jaune  de  Mars. 

Blue. 

Cobalt. 

French  blue. 
Smalt. 

New  blue. 


Yellow. 

Cadmium-yellow. 
Yellow  ochre. 
Roman  ochre. 

Brown. 

Burnt  umber. 
Burnt  Sienna. 


The  following  pigments  were  all  more  or  less  affected;  those 
that  were  very  little  changed  head  the  list,  which  is  arranged  so  as 
to  indicate  the  relative  amounts  of  damage  suffered,  the  most  fugi- 
tive colours  being  placed  at  its  end  : 


Chrome-yellow  becomes  slightly  greenish. 

Red  lead  becomes  slightly  less  orange. 

Naples  yellow  becomes  slightly  greenish  brown. 

Raw  Sienna  fades  slightly  ; becomes  more  yellowish. 
Vermilion  becomes  darker  and  brownish. 

Aureoline  fades  slightly. 

Indian  yellow  fades  slightly. 

Antwerp  blue  fades  slightly. 

Emerald  green  fades  slightly, 

Olive  green  fades  slightly,  becomes  more  brownish. 
Rose  madder  fades  slightly,  becomes  more  purplish. 
Sepia  fades  slightly. 

Prussian  blue  fades  somewhat. 

Hooker’s  green  becomes  more  bluish. 

Gamboge  fades  and  becomes  more  grey. 

Bistre  fades  and  becomes  more  grey. 

Burnt  madder  fades  somewhat. 

Neutral  tint  fades  somewhat. 

Vandyck  brown  fades  and  becomes  more  grey. 

Indigo  fades  somewhat.  . 

Brown  pink  fades  greatly. 


APPENDIX  TO  CHAPTER  VII. 


91 


Violet  carmine  fades  greatly,  becomes  brownish. 

Yellow  lake  fades  greatly,  becomes  brownish. 

Crimson  lake  fades  out. 

Carmine  fades  out. 

To  this  we  may  add  that  rose  madder,  burnt  or  brown  madder, 
and  purple  madder,  all,  are  a little  affected  by  an  exposure  to  sun- 
light for  seventy  hours.  Pale  washes  of  the  following  pigments 
were  completely  faded  out  by  a tnuch  shorter  exposure  to  sunlight : 


Carmine, 

Full  red. 
Dragon’s  blood. 


Italian  pink, 
Violet  carmine. 


Yellow  lake. 
Gall-stone, 
Brown  pink. 


CHAPTER  VIII. 


ON  THE  ABNORMAL  PERCEPTION  OF  COLOUR,  AND 
ON  COLO UR-BLINDNESS. 

AY E have  considered  now,  with  some  detail,  the  various  ' 
ordinary  modes  of  producing  the  sensation  of  colour  ; hut, 
in  order  to  render  our  account  more  complete,  it  is  necessary 
to  touch  on  some  of  the  unusual  or  extraordinary  methods. 

In  every  case  examined  thus  far,  the  sensation  of  colour  was 
generated  by  the  action  on  the  eye  of  coloured  light — that 
is,  of  waves  of  light  having  practically  a definite  length. 
As  colour,  however,  is  only  a seiisation,  and  has  no  existence 
apart  from  the  nervous  organization  of  living  beings,  it  may 
not  seem  strange  to  find  that  it  can  be  produced  by  white 
as  well  as  by  coloured  light,  or  even  that  it  can  be  developed 
in  total  darkness,  without  the  agency  of  light  of  any  kind 
whatever.  If  the  eyes  be  directed  for  a few  moments  to- 
ward a sheet  of  white  paper  placed  on  a black  background 
and  illuminated  by  sunlight,  on  closing  them  and  excluding 
all  light  by  the  hands  or  otherwise,  it  will  be  found  that 
the  absence  of  the  light  does  not  at  once  cause  the  image 
of  the  paper  to  disappear.  After  the  eyes  are  closed  it  will 
still  be  plainly  visible  for  several  seconds,  and  will  at  first 
be  seen  quite  correctly,  as  a white  object  on  a black  ground  ; 
the  colour  with  some  observers  then  changes  to  blue,  green, 
red,  and  finally  back  to  blue,  the  background  remaining  all 
the  while  black.  After  this  first  stage  the  background 
changes  to  white,  the  colour  of  the  sheet  of  paper  appearing 
blue^green,  and  finally  yellow.  Most  of  these  colours  are 


ON  THE  ABNORMAL  PERCEPTION  OF  COLOUR,  ETC.  93 


as  distinct  and  decided  as  those  of  natural  objects.  If  the 
experiment  be  .made  for  a shorter  time,  and  under  a less 
brilliant  illumination,  the  eyes  being  first  well  rested  by 
prolonged  closure,  the  series  of  colours  will  be  somewhat 
different.  Fechner,  Seguin,  and  Helmholtz  observed  that 
the  original  white  colour  passed  rapidly  through  a greenish 
blue  (Seguin,  green)  into  a beautiful  indigo-blue  ; this  af- 
terward. changed  into  a violet  or  rose  tint.  These  colours 
were  bright  and  clear,  afterward  followed  a dirty  or  grey 
orange  ; during  the  presence  of  this  colour  the  background 
changed  from  black  to  white,  and  the  orange  tint  altered 
often  into  a dirty  yellow-green  which  completed  the  series. 
If,  instead  of  employing  white,  a coloured  object  on  a grey 
ground  is  regarded  intently  for  some  time,  the  eyes  will  be 
so  affected  that,  on  suddenly  removing  the  coloured  object, 
the  grey  ground  will  appear  tinged  with  a complementary 


Fig.  26.— Disk  with  Black  and  Fig.  27.— Black  and  White  Spiral 

White  Sectors  for  the  Produc-  on  Disk,  for  the  Production  of 

tion  of  Subjective  Colour.  Subjective  Colour. 


tint  ; for  example,  if  the  object  be  red,  the  after-image  will 
be  bluish  green.  It  is  not  necessary  to  dwell  longer  on 
these  phenomena  at  present,  as  a portion  of  Chapter  XV. 
will  be  especially  devoted  to  them.  In  both  the  cases  men- 
tioned above,  the  colour  develops  itself  after  the  eyes  are 
closed,  or  at  least  withdrawn  from  the  illuminated  surface. 
There  are,  however,  cases  where  very  vivid  colours  can  be 
seen  while  the  eyes  are  exposed  to  full  daylight.  If  a disk 


94 


MODERN  CUROMATICS. 


of  cardboard  painted  with  alternate  white  and  black  sectors, 
like  that  shown  in  Fig.  2G,  be  set  in  rotation  while  exposed 
to  full  daylight,  colours  will  be  seen  after  a few  trials.  It 
will  be  found  that  a certain  rate  of  rotation  communicates 
to  the  disk  a green  hue,  a somewhat  more  rapid  rate  causing 
it  to  assume  a rose  colour.  According  to  Helmholtz,  these 
effects  are  most  easily  attained  by  using  a disk  painted  with 
a black  spiral,  like  that  in  Fig.  27.  These  phenomena  may 
be  advantageously  studied  by  a method  which  was  used  by 
the  author  several  years  ago.  A blackened  disk  with  four 
oj)en  sectors  seven  degrees  in  width  was  set  in  revolution 
by  clockwork,  and  a clouded  sky  viewed  through  it.  With 


Fio.  2S.— Subjective  Colours 
seen  In  Skv,  with  aid  of 
Kotatlng  Disk. 


Fio.  29.— Subjective  Colonrs.  Ulng, 
etc.,  seen  in  Sky  with  aid  of  Ro- 
tating Disk. 


a rate  of  nine  revolutions  per  second,  the  whole  sky  often 
aj)peared  of  a deep  crimson  hue,  except  a small  spot  in  the 
centre  of  the  field  of  view,  which  was  pretty  constantly  yel- 
low. Upon  increasing  the  velocity  to  eleven  and  a half 
revolutions  per  second,  the  central  spot  enlarged  somewhat, 
and  became  coloured  bluish  green,  with  a narrow,  faint,  blue 
border,  indicated  by  the  dotted  line  ; the  rest  of  the  sky 
appeared  purple,  or  reddish  purple.  (See  Fig.  28.)  With 
the  exception  of  fluctuations  in  the  outline  of  the  spot,  this 
appearance  remained  tolerably  constant  as  long  as  the  rate 
of  revolution  was  steadily  maintained  When  the  velocity 


ON  THE  ABNORMAL  PERCEPTION  OF  COLOUR,  ETC.  95 

of  the  disk  was  increased,  the  bluish-green  spot  expanded 
into  an  irregularly  shaped  blue-green  ring,  which  with  a 
rate  of  fifteen  turns  per  second  mostly  filled  the  whole  field 
of  view.  (See  Fig.  29.)  With  higher  rates  all  these  ap- 
pearances vanished,  and  the  sky  was  seen  as  with  the  naked 
eye.*  More  than  one  elaborate  attempt  has  been  made  to 
found  on  phenomena  of  this  class  a theory  of  the  production 
of  colour,  though  it  may  easily  be  shown  that  in  all  such 
cases  the  disk  really  transmits  not  coloured  but  white  light, 
and  that  the  effects  produced  are  due  to  an  abnormal  state 
of  the  retina  caused  by  alternate  exposure  to  light  and 
darkness. 

A current  of  electricity  is  also  capable  of  stimulating 
the  optic  nerve  in  such  a way  that  brilliant  colours  are  per- 
ceived, although  the  experiment  is  made  in  perfect  dark- 
ness. If  the  current  of  a strong  voltaic  battery  be  caused 
to  enter  the  forehead,  and  travel  hence  to  the  hand,  accord- 
ing to  Ritter,  a bright-green  or  bright-blue  colour  is  per- 
ceived, the  hue  depending  on  whether  the  positive  current 
enters  the  hand  or  forehead.  Helmholtz,  in  repeating  this 
operation,  was  conscious  simply  of  a wild  rush  of  colours 
without  order.  The  experiment  is,  however,  interesting  to 
us,  as  proving  the  possibility  of  the  production  of  the  sen- 
sation of  colour  without  the  presence  or  action  of  light. 

Recently  a substance  has  been  discovered  which,  when 
swallowed,  causes  white  objects  to  appear  coloured  greenish 
yellow,  and  coloured  objects  to  assume  new  hues.  Persons 
under  the  influence  of  santonin  cannot  see  the  violet  end  of 
the  spectrum  ; and  this  fact,  with  others,  proves  that  they 
have  become  temporarily  colour-blind  to  violet. 

An  observation  of  Tait’s,  and  others  by  the  author,  have 
shown  that  a shock  of  the  nervous  system  may  produce 
momentarily  colour-blindness  to  green  light.  White  objects 
then  appear  of  a purplish  red,  and  green  objects  of  a much 

* “American  Journal  of  Science  and  Arts,”  September,  1860. 

5 


9G 


MODERN  CHROMATICS. 


duller  green  hue  than  ordinarily.*  These  effects  are  eva- 
nescent, though  quite  interesting,  as  we  shall  see  presently, 
from  a theoretical  point  of  view. 

Investigations  during  the  ])resent  century  have  shown 
that  many  ])ersons  are  horn  with  a deficient  perception  of 
colour.  In  some  the  defect  is  slight  and  hardly  noticeable, 
while  in  others  it  is  so  serious  as  to  lead  to  quite  wonderful 
blunders.  This  imperfection  of  vision  is  often  inherited 
from  a ])arent,  and  may  be  shared  by  several  members  of 
the  same  family.  It  is  remarkable  that  women  are  com- 
paratively free  from  it,  even  when  belonging  to  families  of 
which  the  male  mend)ers  are  thus  affected.  The  occuj)ations 
of  women,  their  attention  to  dress  and  to  various  kinds  of 
handiwork  where  colour  enters  in  as  an  ini])ortant  element, 
seem  to  have  brought  their  sense  for  colour  to  a liigher  de- 
gree of  ])erfection  than  is  the  case  with  men,  who  ordinarily 
neglect  cultivation  in  this  direction.  Out  of  forty-one 
young  men  in  a gymnasium,  Seebeck  found  live  who  were 
colour-blind  ; but  during  his  whole  investigation  he  was 
able  to  learn  of  only  a single  case  wliere  a woman  was  to 
some  extent  similarly  affected.  It  not  unfrequently  happens 
that  persons  with  this  defect  remain  for  years  unconscious 
of  it.  This  was  the  case  with  some  of  the  young  men  in- 
vestigated by  Seebeck  ; and  in  one  remarkable  instance  a 
bystander,  in  attempting  to  l»elp  a colour-blind  person  who 
was  under  investigation,  showed  that  he  was  himself  colour- 
blind, but  belonged  to  another  class  ! The  commonest  case 
is  a deticient  perception  of  red.  Such  persons  make  no  dis- 
tinction between  rose-red  and  bluish-green.  They  see  in 
the  spectrum  only  two  colours,  which  they  call  yellow  and 
blue.  Under  the  name  yellow  they  include  the  red,  orange, 
yellow,  and  green  spaces  : the  blue  and  violet  they  name, 
with  some  correctness,  blue.  In  the  middle  of  the  spectrum 

* “ American  Journal  of  Science  and  Art?,”  January,  ISTV.  A similar 
observation  by  Charles  Pierce  was  communicated  to  the  author  while  this 
work  was  going  through  the  press. 


ON  THE  ABNORMAL  PERCEPTION  OF  COLOUR,  ETC.  97 


there  is  for  them  a neutral  or  grey  zone,  which  has  no 
colour  ; this,  according  to  Preyer,  is  situated  near  the  line 
F.  For  the  normal  eye  it  is  greenish-blue  ; for  them,  white. 
The  extreme  red  of  the  spectrum,  when  it  is  faint,  they  fail 
to  distinguish  ; the  rest  of  the  red  space  appears  to  them  of 
a saturated  but  not  luminous  green  ; the  yellow  space  has 
for  them  a colour  which  we  should  call  bright  green  ; and 
finally,  they  see  blue  in  the  normal  manner.  Maxwell  found 
that  by  the  aid  of  his  disks,  using  only  two  colours,  along 
with  white  and  black,  he  was  able,  by  suitable  variations  in 
their  proportions,  to  match  for  them  any  colour  which  pre- 
sented itself  ; while  the  normal  eye  requires  at  least  three 
such  coloured  disks,  besides  white  and  black.  His  experi- 
ments led  to  the  result  that  persons  of  this  class  perceive 
two  of  the  three  fundamental  colours  which  are  seen  by  the 
normal  eye.  Helmholtz  also  arrived  at  the  same  result.  It 
is  possible  to  render  the  normal  eye  to  some  extent  colour- 
blind to  red  in  the  manner  followed  by  Seebeck  in  1837,  and 
afterward  by  Maria  Bokowa.  These  observers  wore  for 
several  hours  spectacles  provided  with  ruby-red  glasses  ; 
and  this  prolonged  action  of  the  red  light  on  the  eye  finally, 
to  a considerable  extent,  tired  out  the  nerve  fibrils  destined 
for  the  reception  of  red,  so  that  on  removing  the  glasses  they 
saw  in  the  spectrum  only  two  colours.  The  second  observer 
called  them  yellow  and  blue.  Furthermore,  the  extreme 
red  end  of  the  spectrum  was  not  visible  to  her,  just  as  is  the 
case  with  those  who  are  actually  blind  to  red  ; all  red  objects 
appeared  to  her  yellow,  and  dark  red  was  not  distinguishable 
from  dark  green  or  brown. 

Dalton,  the  celebrated  English  chemist,  suffered  from 
this  defect  of  vision,  and  was  the  first  to  give  an  accurate 
description  of  it  ; hence  this  affection  is  sometimes  named 
after  him,  Daltonism.  It  is  very  remarkable  that,  accord- 
ing to  tlie  observations  of  Schelske  and  Helmlioltz,  even  in 
the  noianal  eye  there  are  portions  which  are  naturally  colour- 
blind to  red,  and  when  this  zone  of  the  eye  is  used  the  same 


98 


MODERN  CHROMATICS. 


mistakes  in  matching  colours  are  made.  Such  exj^eriments 
are  somewhat  difficult  to  make  without  considerable  prac- 
tice, as  it  is  necessary  that  the  colored  objects  should  he 
viewed,  not  directly,  hut  hy  the  eye  turned  aside  somewhat. 
There  is  a simple  means  hy  which  persons  who  are  colour- 
blind to  red  can  to  some  extent  help  themselves,  and  prevent 
the  occurrence  of  coarse  chromatic  blunders,  such  as  con- 
fusing red  with  green.  Green  glass  does  not  transmit  red 
light  ; hence,  on  vieAving  green  and  red  objects  through  a 
plate  of  this  glass  it  will  he  found,  even  hy  persons  who  are 
colour-blind,  that  the  red  objects  appear  much  more  dark- 
cnetl  than  those  which  are  green.  On  the  other  hand,  a red 
glass  will  cause  green  objects  to  apj)ear  darker,  but  will  not 
atfect  the  luminosity  of  those  having  a tint  similar  to  itself. 
"Idle  exact  tints  of  the  glasses  are  important,  and  they  should 
of  course  be  selected  with  the  aid  of  a normal  eye. 

The  kind  of  colour-blindness  just  described  is  rather 
common,  and  it  has  been  estimated  that  in  England  about 
one  person  in  eighteen  is  more  or  less  affiictcd  with  it.  AVe 
pass  on  now  to  the  consideration  of  a class  of  cases  that  is 
more  rare.  1 Arsons  belonging  to  this  second  class  see  only 
two  colours  in  the  s])ectrum,  which  they  call  red  and  blue. 
They  set  the  greatest  luminosity  in  the  spectrum  in  the 
yellow  sjKXce,  as  is  done  by  the  normal  eye  ; and  they  easily 
distinguish  between  red  and  violet,  but  confuse  green  witli 
yellow  and  blue  with  red.  In  two  cases  examined  by  Preyer, 
yellow  appeared  to  them  as  a lu*ight  red  ; this  same  observer 
also  found  that  in  the  spectrum,  near  the  line  h,  the  two 
colours  into  which  theV  divided  the  spectrum  were  separated 
by  a small  neutral  zone,  which  was  for  them  identical  with 
grey.  A sufficient  number  of  observations  have  not  been 
accumulated  to  furnish  means  of  ascertaining  with  certainty 
the  exact  nature  of  the  difficulty  under  which  they  labour, 
though  it  is  probable  that  they  are  colour-blind  to  green 
light.  There  are  also  observations  on  record  of  cases  of 
temporary  colour-blindness  of  a third  kind,  where  the  violet 


ON  THE  ABNORMAL  PERCEPTION  OF  COLOUR,  ETC.  99 

end  of  the  spectrum  was  seen  shortened  to  a very  remark- 
able extent ; and  if  it  should  prove  that  the  cause  was  of  a 
nervous  character,  rather  than  due  to  a deeper  yellow  col- 
ouration of  the  axial  portions  of , the  retina,  this  would 
demonstrate  the  existence  of  violet  colour-blindness. 

The  subject  of  colour-blindness  is  one  of  considerable 
importance  from  a practical  point  of  view,  and  this  defect 
has  no  doubt  been  the  occasion  of  railroad  accidents.  In 
1873-75  Dr.  Favre,  in  France,  examined  one  thousand  and 
fifty  railroad  ofiacials  of  various  grades,  and  found  among 
them  ninety-eight  persons  who  were  colour-blind — that  is, 
9*33  per  cent.  In  1876  Professor  Holmgren,  in  Sweden, 
examined  the  entire  personnel  of  the  Upsala-Gefle  line,  and 
out  of  two  hundred  and  sixty-six  persons  ascertained  that 
thirteen  were  colour-blind.  These  were  found  in  every 
grade  of  the  service,  many  of  them  being  required  daily  to 
make  use  of  coloured  signals.  It  is  singular  that  in  no  case 
nad  there  been  previously  any  suspicion  of  the  existence  of 
the  defect.  For  further  information  with  regard  to  the 
practical  side  of  this  matter,  the  reader  is  referred  to  the 
essay  of  Holmgren,  which  will  be  found  in  the  Smithsonian 
Report  for  1877  : a French  translation  also  exists. 

In  concluding  this  subject,  it  may  not  be  amiss  to  allude 
to  the  very  remarkable  case  described  by  Huddart,  of  a 
shoemaker,  an  intelligent  man,  where  only  a trace  of  the 
power  to  distinguish  colours  seemed  to  remain.*  According 
to  the  observations,  he  was  colour-blind  to  both  red  and 
green,  and  in  general  seems  to  have  had  hardly  any  percep- 
tion of  colour,  as  distinguished  from  light  and  shade.  Curi- 
ously enough,  recent  observations  of  Woinow  show  that 
even  in  the  normal  eye  there  is  a condition  like  this  at  the 
farthest  limit  of  the  visible  field  of  view  ; here  all  distinc- 
tions of  colour  vanish,  and  objects  look  merely  white  or 
black,  or  grey.  It  is  probable  that  between  the  case  of 


Philosophical  Transactions,”  Ixvii. 


100 


MODERN  CHROMATICS. 


Harris,  just  mentioned,  and  that  of  a normal  eye  possessed 
of  the  maximum  power  of  perceiving  and  distinguishing 
colours,  a great  number  of  intermediate  gradations  will  be 
found  to  exist.  Slight  chromatic  defects  of  vision  generally 
receive  no  attention,  or  are  explained  in  some  other  way. 
Tlie  writer  recalls  the  ease  of  an  excellent  physicist  who 
for  many  years  had  a half  susj)icion  that  he  was  to  some 
extent  colour-blind,  but  rather  preferred  to  explain  the  dis- 
crepancies by  the  assumption  of  a difference  in  nomencla- 
ture. Taking  up  the  matter  at  last  seriously,  he  made  an 
investigation  of  his  own  case,  and  found  that  he  actually 
was  to  some  extent  colour-blind  to  red.  It  has  been  sug- 
gested that  the  very  inferior  colouring  of  some  otherwise 
great  artists  can  be  accounted  for  by  supposing  them  to 
have  been  atfecte<l  with  partial  colour-blindness  ; this  idea 
is  plausible,  but,  as  it  ap})cars  to  us,  totally  without  proof. 
There  are  great  numbers  of  persons  who  are  able  to  hear 
distinctly  all  the  notes  employed  in  music,  Avho  yet  have  no 
talent  for  it  and  no  enjoyment  of  it.  On  the  other  hand, 
we  know  of  cases  of  persons  who  from  infancy  have  been 
atHicted  with  j)artial  deafness,  and  have  nevertheless  been 
musicians,  and  even  composers,  ft  is  the  same  in  painting 
as  in  music  : the  possession  of  a perfect  organ  is  not  by  any 
means  the  first  necessity,  and  it  can  be  jtroved  that  even 
artists  who  are  actually  coloui -blind  to  red  may  still,  with 
but  slight  external  aid,  produce  jiaiiitings  which  are  univer- 
sally prized  for  their  beautiful  colouring.  Their  field  of 
operations  is  of  course  more  restricted,  and  they  are  com- 
])elled  to  avoid  certain  chromatic  combinations.  During 
the  evening,  by  gas-  or  lamp-light,  we  are  all  somewhat  in 
the  condition  of  persons  who  are  colour-blind  to  violet  ; but 
yet,  with  precautions  and  some  patience,  it  is  possible  to 
execute  works  in  colour,  even  at  this  time,  which  afterward 
stand  the  test  of  daylight.  It  would  appear  probable,  then, 
that  the  difficulty  with  the  inferior  colourists  above  alluded 
to  was  not  so  much  anatomical  or  physiological  as  psychical. 


ON  THE  ABNORMAL  PERCEPTION  OF  COLOUR,  ETC.  IQl 

According  to  a theory  recently  proposed  by  Hugo  Mag- 
nus, our  sense  for  colour  has  been  developed  during  the  last 
four  or  five  thousand  years  ; previous  to  this  period  it  is 
assumed  that  our  race  was  endowed  only  with  a perception 
of  light  and  shade.  The  evidence  which  is  offered  is  of  a 
philological  character,  and  tends  to  show  that  the  ancients 
either  distinguished  or  described  colours  less  accurately 
than  the  moderns.  The  same  kind  of  reasoning  might  be 
applied  to  proving  that  children  at  the  present  day  have 
but  little  power  of  distinguishing  tints,  as  they  usually 
scarcely  notice  any  but  the  most  intense  colours.  When, 
however,  we  study  the  prehistoric  races  at  present  existing 
on  the  globe,  and  living  in  the  same  style  as  their  ancestors, 
we  find  them  quite  capable  of  discriminating  colours,  and 
often  very  fond  of  them.  Going  many  steps  lower,  we 
meet  with  animals  that  have  the  power  of  perceiving  and 
even  imitating  a series  of  colours  with  accuracy.  This  is 
the  case  with  the  chameleon,  as  shown  by  P.  Bert,  and  also, 
according  to  Pouch et  and  A.  Agassiz,  with  certain  kinds  of 
flounders.  The  skin  of  the  chameleon  is  provided  with  an 
immense  number  of  minute  sacs  filled  with  red,  yellow,  and 
black  liquids  ; the  animal  has  the  power  of  distending  these 
star-like  vesicles  at  pleasure,  and  thus  adjusts  its  colour  in 
a few  minutes  (after  a series  of  trials)  so  as  to  match  that 
of  the  surface  on  which  it  is  placed.  Its  chromatic  scale 
covers  red,  orange,  yellow,  and  olive-green,  and  the  mix- 
tures of  these  colours  with  black,  which  includes  of  course 
an  extensive  series  of  browns.  The  olive-green  is  made  by 
distending  the  yellow  and  black  sacs,  the  effect  being  simi- 
lar to  that  obtained  by  combining  a black  and  yellow  disk. 
(See  Chapter  XII.) 

Corresponding  to  this,  A.  Agassiz  has  often  noticed, 
when  a young  flounder  was  transferred  from  a jar  imitating 
in  colour  a sandy  bottom  to  one  of  a dark-chocolate  hue, 
that  in  less  than  ten  minutes  the  black-pigment  cells  would 
obtain  a great  preponderance,  and  cause  it  to  appear  wholly 


102 


MODERN  CHROMATICS. 


unlike  the  yellowish-grey  speckled  creature  which  a few 
moments  before  had  so  perfectly  simulated  the  appearance 
of  sand.  When  removed  to  a 'gravelly  bottom,  the  S20Ots 
on  the  side  became  prominent. 

If  our  sense  for  light  and  shade  is  old,  but  that  for 
colour  recent  and  still  undergoing  development,  we  should 
perhaps  expect  that  it  would  require  more  time  to  recog- 
nize colour  than  appearances  dependent  simply  on  light 
and  shade  ; but,  according  to  the  experiments  of  the  writer, 
forty  billionths  of  a second  answers  as  well  for  one  as  for 
the  other  act  of  perception.* 

In  closing  this  chapter,  it  may  be  well  to  mention  a very 
simple  but  beautiful  experiment,  by  which  we  all  can  easily 
place  ourselves  in  a condition  somewhat  like  that  of  Harris, 
where  all  or  nearly  all  sensation  of  colour  had  vanished. 
If  some  carbonate  of  soda  be  ignited  in  the  flame  of  a Bun- 
sen burner,  it  will  furnish  an  abundance  of  pure  homoge- 
neous light  of  an  orange-yellow  hue.  This  light  is  quite 
bright  enough  to  illuminate  objects  in  a darkened  room, 
but  all  distinctions  of  .colours  vanish,  light  and  shade  only 
remain  in  2T.  A red  rose  exhibits  no  more  colour  than  its 

O 

leaves  ; gn-yly  painted  strips  of  paper  show  only  as  black  or 
white  or  grey  ; their  colours  can  not  even  be  guessed  at. 
The  human  face  divested  of  its  natural  colour  assumes  an 
appearance  which  is  repulsive,  and  the  eye  in  the  absence 
of  colour  dwells  on  slight  defects  in  the  clearness  and 
smoothness  of  the  complexion.  If  now  an  ordinary  gas- 
burner  be  placed  near  the  soda  flame,  and  allowed  at  first 
to  burn  with  only  a small  flame,  objects  will  resume  their 
natural  tints  to  some  slight  extent,  and  begin  again  faintly 
to  clothe  themselves  with  pleasant  hues,  which  will  deepen 
as  more  light  is  furnished,  till  they  finally  seem  fairly  to 

* The  amount  of  time  necessary  for  vision.  “ American  Journal  of  Sci- 
ence,” September,  1871. 


APPENDIX  TO  CHAPTER  VIII. 


103 


glow  with  radiant  beauty.  Those  who  have  never  wit- 
nessed an  experiment  of  this  kind  have  but  little  conception 
how  great  would  be  to  us  the  loss  of  our  sense  for  colour, 
or  how  dreary  the  world  would  seem,  divested  of  the  fasci- 
nating charm  which  it  casts  over  all  things. 


APPElirDIX  TO  CHAPTER  VIII.  * 

Maxwell  has  published  an  account  of  his  rather  elaborate 
examination  of  the  case  of  one  of  his  students,  who  was  colour- 
blind to  red.*  An  apparatus  was  employed  by  which  the  pure 
colours  of  the  spectrum  could  be  mixed  in  any  proportion ; these 
colours  were  then  mingled  by  the  colour-blind  person,  in  such  a 
proportion  as  to  produce  to  his  eyes  the  effect  of  white.  In  this 
way  the  following  equation  was  obtained : 33*7  green  -p  33-1  blue 
= white.  Maxwell  then,  employing  the  same  colours,  obtained  his 
own  or  a normal  equation,  which  was:  26  green  + 37”4  blue  -t-  22-6 
red  = white.  If  we  combine  these  equations  by  subtraction,  we 
obtain : 22-6  red  — 7*7  green  -t-  4-3  blue  = D ; D being  the  missing 
colour  not  perceived  by  the  colour-blind.  The  sensation,  then. 


y 

— 

K 

ABCDEF  G H 

Fig.  30,— Curves  of  a Colour-blind  Person.  (Maxwell.) 


which  Maxwell  had  in  addition  to  those  of  the  colour-blind  person 
was  somewhat  like  that  of  a full  red,  but  different  from  it  in  that 
the  full  red  was  mixed  with  7‘7  green,  which  had  to  be  removed 
from  it,  and  4-3  of  blue  substituted.  The  missing  colour,  then,  ac- 
cording to  this  experiment,  was  a crimson-red.  Even  normal  eyes 
vary  a little,  and,  if  this  examination  had  been  made  by  Maxwell’s 


* “ Philosophical  Transactions  ” for  1860,  vol.  cl.,  p.  78. 


104 


MODERN  CHROMATICS. 


assistant  (observer  K),  the  result  would  have  been  a red  mixed  with 
less  blue,  consequently  a colour  much  more  like  the  red  of  the 
spectrum.  From  experiments  of  this  kind  Maxwell  was  able  to 
construct  the  curves  of  intensity  of  the  two  fundamental  colours 
which  are  ])erceived  by  those  who  are  colour-blind  to  red ; these 
curves  are  shown  in  Fig.  30.  The  letters  A,  B,  0,  D,  etc.,  mark 
the  positions  of  the  fixed  lines  in  the  solar  spectrum ; the  curved 
line  marked  GR  exhibits  the  intensity  of  the  green  element,  the 
line  marked  BL  that  of  the  blue  or  violet.  It  will  be  noticed  that 
the  green  sensation  attains  its  maximum  about  lialf  way  between 
the  lines  D and  E,  that  is,  in  the  yellowish  green  ; while  the  high- 
est point  of  the  other  curve  is  about  half  way  between  F and  G, 
that  is,  in  the  blue  s{)ace.  Maxwell  also  constructed  similar  inten- 
sity curves  for  a normal  eye;  they  are  represented  in  Fig.  31,  the 


C D E F G 


curve  for  red  being  indicated  by  a heavy  line,  the  others  as  above. 
The  green  and  blue  curves  have  about  the  same  disposition  as  with 
the  colour-blind  person,  while  the  red  attains  its  maximum  between 
C and  D,  but  nearer  D — that  is,  in  the  red-orange  space. 

A set  of  observations  was  also  made  by  Maxwell  on  the  same 
colour-blind  gentleman,  with  the  aid  of  coloured  disks  in  rapid  ro- 
tation ; and,  from  the  colour  equations  thus  obtained,  the  positions 
of  the  colours  perceived  by  him  were  laid  down  in  Newton’s  dia- 
gram, in  a manner  similar  to  that  explained  in  the  appendix  to 
Chapter  XIV.  In  Fig.  32,  V shows  the  position  assumed  for  red 
or  vermilion  ; TJ,  that  of  ultramarine- blue ; and  G,  that  of  emerald- 


APPENDIX  TO  CHAPTER  VIIL 


105 


green.  They  are  placed  according  to  Maxwell’s  method,  at  the 
three  angles  of  an  equilateral  triangle.  W would  be  the  position  of 
white  for  a normal  eye,  and  Y that  of  chrome-yellow.  D is  the 
position  of  the  defective  colour,  which  Maxwell  was  able  to  imitate 
by  mingling,  by  the  method  of  rotating  disks,  86  parts  of  vermilion 
and  14  of  ultramarine-blue.  A line  drawn  from  D through  W con- 
tains along  its  length  the  various  shades  of  grey  and  the  white  of 
the  colour-blind.  The  grey  which  they  perceive  when  green  and 


blue  are  mixed  lies  at  w;  the  white  of  white  paper,  i.  e.,  a more 
luminous  grey,  was  on  the  same  line  but  considerably  farther  along 
outside. 

It  may  perhaps  be  as  well,  to  add  to  the  above  one  or  two  re- 
marks concerning  the  construction  of  Newton’s  diagram  for  the 
colour-blind.  Let  us  suppose  that  the  pure  colours  of  the  spectrum 
are  employed,  and  that  the  missing  colour  is  the  fundamental  red ; 
we  then  place  the  fundamental  green  at  G,  Fig.  33,  the  fundamental 
blue  or  violet  at  U,  and  the  missing  red  at  D.  Then  along  the  line 
U G will  lie  mixtures  of  blue  and  green,  and  at  w will  be  the  white 
of  the  colour-blind  person.  Along  the  line  D G will  be  situated 
various  shades  of  green,  from  dark  green  to  bright  green,  the  latter 
colour  predominating  as  we  approach  G.  Along  the  line  I)  U we 
shall  have  various  shades  of  blue,  from  bright  blue  to  dark  blue, 
the  colour  being  very  dark  near  D and  very  bright  near  U.  A line 
like  the  dotted  one  (Fig.  33)  will  conhiin  various  shades  of  green, 
from  light  green  to  dark  green,  but  none  of  them  so  intense  as 


U 


Fig.  82. — Newton’s  Diagram  for  a Person 
Colour-blind  to  Ked.  (Maxwell.) 


Fig.  33. — Newton’s  Diagram  for  a Per- 


son Colour-blind  to  the  Fundamental 
Eed. 


106 


MODERN  CHROMATICS. 


those  situated  along  the  line  D G ; in  other  words,  they  all  will  be 
mingled  with  what  the  colour-blind  call  white. 

If  the  defective  colour-sensation  is  supposed  still  red,  but  to  be 
only  partially  absent,  the  diagram  takes  the  form  indicated  in  Fig.  34 ; 
that  is,  red,  instead  of  occupying  the  position  at  one  of  the  angles 
of  an  equilateral  triangle,  will  be  moved  up  toward  the  centre  to  R'. 
White  will  also  be  shifted  from  W to  and  the  white  of  a person 
thus  affected  would  appear  to  the  normal  eye  of  a somewhat  green- 
ish-blue hue.  Between  D and  B'  lie,  so  to  speak,  mixtures  of  red 
with  darkness,  and  along  the  line  R'  G will  be  various  mixtures 
of  red  and  green,  in  which,  according  to  a normal  eye,  the  green 
element  quite  predominates ; that  is  to  say,  their  orange  is  more 


G- 


Fio.  84.— Ncu'ton''s  Dlnsram  for  a Per- 
eoD  partially  Colour-blind  to  Red. 


G 


like  our  yellow,  their  yellow  like  our  greenish  yellow,  etc.  Along 
the  line  R'  U will  bo  their  mixtures  of  red  and  blue,  or  a series  of 
purples,  which  will  be  more  bluish  than  ours. 

The  condition  of  the  normal  eye  by  lamp-light  is  shown  in  Fig. 
35.  The  blue  or  violet  is  moved  from  U,  its  ])osition  by  daylight, 
up  to  u ; white  is  moved  from  W to  w — that  is,  into  a region  that 
would  be  called  by  daylight  yellow.  Yellow  itself,  Y,  is  not  far 
from  this  new  representative  of  white,  and  consequently  by  candle- 
light appears  always  whitish.  In  the  purples,  along  the  line  R «, 
the  red  element  predominates;  and  in  the  mixtures  of  green  and 
blue,  along  the  line  G iq  the  green  constituent  has  the  upper  hand. 

If  we  were  colour-blind  to  every  kind  of  light  except  red,  then 
the  colour  diagram  would  assume  a form  similar  to  that  shown  in 
Fig.  36,  D representing  the  darkest  red  perceptible  to  eyes  so  consti- 
tuted. This  sensation  would  be  brought  about  by  pure  feeble  red 


APPENDIX  TO  CHAPTER  VIII.  . 107 

light,  or  by  a mixture  of  intense  green  and  blue  light,  or  by  either 
of  the  latter.  As  we  advance  from  D toward  R,  the  red  light 
gains  in  brightness,  and  out  at  w becomes  very  bright  and  stands 
for  white.  When  a red  glass  is  held  before  the  eyes,  something 
approximating  to  this  kind  of  vision  is  produced.  . 

^ Qt 


Fig.  36.— Newton’s  Diagram  for  Persons  Colour-blind  to  Green  and  Violet. 


( IIAPTKK  IX. 

THE  COLOUR  THEORY  OF  YOUNCr  AND  HELMHOLTZ. 

It  is  well  known  to  painters  that  api)roxiinate  represen- 
tations of  all  colours  can  he  prodiicetl  hy  the  use  of  very 
few  pi^nients.  Three  pigments  or  coloured  j)Owders  ^^ill 
sutlice,  a red,  yellow,  and  a blue  ; for  example,  crimson- 
lake,  i^amboge,  and  Prussian  blue.  I he  red  and  yellow 
minified  in  various  pro])ortions  will  turnish  different  shades 
of  orange  and  orange-yellow  ; the  blue  ami  yellow  will 
give  a great  variety  of  greens  ; the  re<l  and  blue  all  tlie 
purple  and  violet  hues.  There  have  been  instances  of 
painters  in  water-colours  who  used  only  these  three  i)ig- 
ments,  adding  lampblack  for  the  ])urpose  of  darkening  them 
and  obtaining  the  browns  and  greys.  Now,  though  it  is 
not  possible  hi  this  way  to  obtain  as  lirilliant  rejiresentatives 
of  the  hues  of  nature  as  with  a less  economical  palette,  yet 
substitutes  of  a more  or  less  satisfactory  character  can  actu- 
ally be  produced  in  this  manner.  These  facts  have  been 
known  to  painters  from  the  earliest  ages,  and  furnished  the 
foundation  for  the  so-called  theory  of  three  primary  colours, 
red,  yellow,  and  blue.  The  most  distinguished  defender  in 
modern  times  of  this  theory  was  Sir  David  Brewster,  so 
justly  celebrated  for  his  many  and  brilliant  optical  discov- 
eries* He  maintained  that  there  were  three  original  or  fun- 
damental kinds  of  light,  red,  yellow,  and  blue,  and  that  by 
their  mixture  in  various  proportions  all  other  kinds  of  col- 
oured light  were  produced,  m the  manner  just  described  for 
pigments.  Brewster  in  fact  thought  he  had  demonstrated 


THE  COLOUR  THEORY  OF  YOUNG  AND  HELMHOLTZ.  109 

the  existence  in  the  spectrum  itself  of  these  three  sets  of 
fundamental  rays,  as  well  as  the  absence  of  all  others  ; and 
his  great  reputation  induced  most  physicists  for  more  than 
twenty  years  to  adopt  this  view,  Airy,  Melloni,  and  Draper 
alone  dissenting.  This  theory  of  the  existence  of  three 
fundamental  kinds  of  light,  red,  yellow,  and  blue,  is  found 
in  all  except  the  most  recent  tfext  books  on  physics,  and  is 
almost  universally  believed  by  artists.  Nevertheless,  it 
will  not  be  difficult  to  show  that  it  is  quite  without  founda- 
tion. If  we  look  at  the  matter  from  a theoretical  point  of 
view,  we  reach  at  once  the  conclusion  that  it  can  not  be 
true,  because  outside  of  ourselves  there  is  no  such  thing  as 
colour,  which  is  a mere  sensation  that  varies  with  the 
length  of  the  wave  producing  it.  Outside  of  and  apart  from 
ourselves,  light  consists  only  of  waves,  long  and  short — in 
fact,  of  mere  mechanical  movements  ; so  that  Brewster’s 
theory  would  imply  that  there  were  in  the  spectrum  only 
three  sets  of  waves  having  three  different  lengths,  which 
we  know  is  not  the  case.  If  we  take  up  the  matter  experi- 
mentally, we  meet  with  no  better  result.  According  to  the 
theory  now  under  consideration,  green  light  is  produced  by 
mixing  blue  and  yellow  light.  This  point  can  be  tested 


Fig.  37. — Maxwell’s  Disks.  Blue  and  Yellow  Disks  in  the  Act  of  being  combined. 

with  Maxwell’s  coloured  disks.  A circular  disk,  painted 
with  chrome-yellow  and  provided  with  a radial  slit,  is  to  be 
combined  with  one  prepared  in  the  same  way  and  painted 
with  ultramarine-blue.  Fig.  37  shows  the  separate  disks, 
and  in  Fig.  38  they  are  seen  in  combination.  If  the  com- 
pound disk  be  now  set  in  quite  rapid  rotation,  the  two  kinds 


110 


MODERN  CHROMATICS. 


of  coloured  light  will  be  mingled,  and  the  resultant  tint  can 
be  studied.  It  will  not  be  green,  but  yellowish  grey  or 


Fig.  88  — Blue  and  Yellow  Disks  in  Combination. 

reddish  grey,  according  to  the  proportions  of  the  two  col- 
ours. 1 hese  disks  of  Maxwell  are  ingeniously  contrived  so 
as  to  allow  the  experimenter  to  mingle  the  two  colours  in 


Fig.  89.— Apparatus  of  Lambert  for  mixing  Coloured  Light. 


any  desired  proportion  ; but,  vary  the  proportions  as  we 
may,  it  is  impossible  to  obtain  a resultant  green  hue,  or  in- 


THE  COLOUR  THEORY  OF  YOUNG  AND  HELMHOLTZ.  HI 

deed  anything  approaching  it.  Another  way  of  making 
this  experiment  is  simply  to  use  a fragment  of  window-glass 
of  good  quality,  as  was  done  by  Lambert  and  Helmholtz. 
This  apparatus  is  shown  in  Fig.  39.  The  glass  is  supported 
in  a vertical  position  about  ten  inches  above  a board  painted 
black,  and  on  either  side  of  it  are  placed  the  coloured 
papers.  The  blue  paper  is  seeh  directly  through  the  glass, 
while  the  light  from  the  yellow  paper  is  first  reflected  from 
the  glass  and  then  reaches  the  eye.  The  result  is  that  the 
two  images  are  seen  superimposed,  as  is  indicated  in  Fig. 
40.  The  relative  luminosity  or  brightness  of  the  two  im- 


Fig.  40.— Eesult  furnislied  by  the  Apparatus. 

ages  can  be  varied  at  will  ; for  instance,  moving  the  papers 
further  apart  causes  the  blue  to  predominate,  and  bringing 
them  nearer  together  produces  the  reverse  effect.  In  this 
manner  the  resultant  tint  may  be  made  to  run  through  a 
variety  of  changes,  which  will  entirely  correspond  to  those 
obtained  with  the  two  circular  disks  ; but,  as  before,  no 
tendency  to  green  is  observed.  Helmholtz  has  pushed  this 
matter  still  further,  and  has  studied  the  resultant  hues  pro- 
duced by  combining  together  the  pure  colours  of  the  spec- 
trum. The  following  experiment,  which  is  easy  to  make, 
will  give  an  idea  of  the  mode  of  proceeding  : A blackened 
screen  of  pasteboard  is  pierced  with  two  narrow  slits,  ar- 
ranged like  those  in  Fig.  41.  The  light  from  a window  is 
allowed  to  shine  through  the  two  slits  and  to  fall  on  a prism 
of  glass  placed  just  before  the  eye,  and  distant  from  the 
slits  about  a metre.  Then,  as  would  be  expected,  each  slit 


112 


MODERN  CHROMATICS. 


furnishes  a prismatic  spectrum,  and  owing  to  the  disposition 
of  the  slits,  the  two  spectra  will  overlap  as  shown  in  Fig. 
42,  which  represents  the  red  space  of  one  spectrum  falling 


Kto.  41.— Two  Slits  arrangtd  for  uii.xln"  Two  Spectra. 

on  the  green  space  of  its  companion.  I>y  moving  the  slits  fur- 
ther apart  or  nearer  together,  all  the  dilferent  kinds  of  light 
which  the  spectrum  contains  may  thus  be  mingled.  Using 
a more  retined  apparatus,  Helmholtz  jiroved  that  the  union 
of  the  pure  blue  with  the  pure  yellow  light  of  the  spectrum 
[troduced  in  the  eye  the  sensation,  not  of  green,  but  of 
white  light.  Other  highly  interesting  results  were  also 
obtained  by  him  during  this  investigation  ; these  will  be 


Fig.  42. — Two  Overlappingr  Spectra.  Red  and  Green  are  mixed,  also  Violet  and  Blue,  etc. 

considered  in  the  following  chapter,  but  in  the  mean  while 
it  is  evident  that  this  total  failure  of  blue  and  yellow  light 
to  produce  by  their  mixture  green  light  is  necessarily  fatal 


THE  COLOUR  THEORY  OF  YOUNG  AND  HELMHOLTZ.  H3 

to  the  hypothesis  of  Brewster.  Helmholtz  also  studied  the 
nature  of  the  appearances  which  misled  the  great  English 
optician,  and  showed  that  they  were  due  to  the  fact  that  he 
had  employed  an  impure  spectrum,  or  one  not  entirely  free 
from  stray  white  light. 

As  has  been  remarked  above,  there  can  be  in  an  objec- 
tive sense  no  such  thing  as  thfee  fundamental  colours,  or 
three  primary  kinds  of  coloured  light.  In  a totally  differ- 
ent sense,  however,  something  of  this  kind  is  not  only  pos- 
sible, but,  as  the  recent  advances  of  science  show,  highly 
probable.  We  have  already  seen  in  a previous  chapter  that 
in  the  solar  spectrum  the  eye  can  distinguish  no  less  than 
a thousand  different  tints.  Every  small,  minute,  almost 
invisible  portion  of  the  retina  of  the  eye  possesses  this 
power,  which  leads  us  to  ask  whether  each  atom  of  the  ret- 
ina is  supplied  with  an  immense  number  of  nerve  fibrils  for 
the  reception  and  conveyance  of  this  vast  number  of  sensa- 
tions. The  celebrated  Thomas  Young  adopted  another 
view  : according  to  him,  each  minute  elementary  portion  of 
the  retina  is  capable  of  receiving  and  transmitting  three 
different  sensations  ; or  we  may  say  that  each  elementary 
portion  of  its  surface  is  supplied  with  three  nerve  fibrils, 
adapted  for  the  reception  of  three  sensations.  One  set  of 
these  nerves  is  strongly  acted  on  by  long  waves  of  light, 
and  produces  the  sensation  we  call  red  ; another  set  re- 
sponds most  powerfully  to  waves  of  medium  length,  produc- 
ing the  sensation  which  we  call  green  ; and  finally,  the  third 
set  is  strongly  stimulated  by  short  waves,  and  generates 
the  sensation  known  as  violet.  The  red  of  the  spectrum, 
then,  acts  powerfully  on  the  first  set  of  these  nerves  ; but, 
according  to  Young’s  theory,  it  also  acts  on  the  two  other 
sets,  but  with  less  energy.  The  same  is  true  of  the  green 
and  violet  rays  of  the  spectrum  : they  each  act  on  all  three 
sets  of  nerves,  but  most  powerfully  on  those  especially 
designed  for  their  reception.  All  this  will  be  better  un- 
derstood by  the  aid  of  the  accompanying  diagram,  which  is 


MODERN  CHROMATICS. 


114 

taken  from  Helmholtz’s  great  work  on  ‘‘  Physiological  Op- 
tics.”  In  Fig.  43,  along  the  horizontal  lines  1,  ^ aie 


placed  the  colours  of  the  spectru.n  properly  arranged  a^ 
the  carves  above  them  indicate  the  degree  to  which  the 
three  kinds  of  nerves  are  acted  on  by  these  colours. 
we  see  that  nerves  of  the  first  kind  are  powerfit  y stim  - 
lated  by  rod  light,  are  much  less  aftected  by  yelloM,  still 
lels  by^reen,  mid  very  little  by  violet  light  >erves  of 
,be  second  kind  are  much  atfected  by  green  'g');’  ^ 
yellow  and  blue,  and  still  less  by  red  and  violet.  Ihe  thud 
kind  of  nerves  answer  readily  to  violet  light,  and  are  suc- 
cessively less  affected  by  other  kinds  of  light  in  ‘he  foUow- 
h.cr  order : blue,  green,  yellow,  orange,  red.  The  next 
point  in  the  theory  is  that,  if  all  three  sets  of  nerves  are 
simultaneously  stimulated  to  about  the 

sensation  which  we  call  white  will  be  produced.  ^re 

the  main  points  of  Young’s  theory,  which  was  published  as 

L 180S,  ™a  Miy  i.  1*07. 

W thin  the  last  few  years  been  called  to  it  by  Helmholtz 
and  it  is  mainly  owing  to  his  labours  and  those  of 
“hat  it  now  cmnmands  such  respectful  attention  Befo  e 
miking  an  examination  of  the  evidence  on  which  it  rests. 


THE  COLOUR  THEORY  OP  YOUNG  AND  HELMHOLTZ.  115 

and  of  its  applications,  it  may  be  well  to  remember,  as 
Helmholtz  remarks,  that  the  choice  of  these  three  particular 
colours,  red,  green,  and  violet,  is  somewhat  arbitrary,  and 
that  any  three  could  be  chosen  which  when  mixed  together 
would  furnish  white  light.  If,  however,  the  end  and  mid- 
dle colours  of  the  spectrum  (red,  violet,  and  green)  are  not 
selected,  then  one  of  the  three  must  have  two  maxima,  one 
in  the  red  and  the  other  in  the  violet  ; which  is  a more 
complicated,  but  not  an  impossible  supposition.  The  only 
known  method  of  deciding  this  point  is  by  the  investigation 
of  those  persons  who  are  colour-blind.  In  the  last  chapter 
it  was  shown  that  the  most  common  kind  of  this  affection 
is  colour-blindness  to  red,  which  indicates  this  colour  as 
being  one  of  the  three  fundamental  sensations.  But,  if  we 
adopt  red  as  one  of  our  three  fundamental  colours,  of 
necessity  the  other  two  must  be  green  and  violet  or  blue- 
violet.  Red,  yellow,  and  blue,  for  example,  will  not  pro- 
duce white  light  when  mingled  together,  nor  will  they 
under  any  circumstances  furnish  a green.  Red,  orange, 
and  blue  or  violet  would  answer  no  better  for  a fundamental 
triad.  In  the  preceding  chapter  it  was  also  shown  that 
colour-blindness  to  green  exists  to  some  extent,  though  by 
no  means  so  commonly  as  the  other  case.  Hence,  th*us  far, 
the  study  of  colour-blindness  has  furnished  evidence  in 
favour  of  the  choice  of  Young,  and  its  phenomena  seem 
explicable  by  it. 

Let  us  now  examine  the  explanation  which  the  theory 
of  Young  furnishes  of  the  production  of  the  following 
colour- sensations,  which  are  not  fundamental,  viz.  : 


Orange-red, 

Red-orange. 

Orange-yellow. 


Yellow. 

Greenish-yellow. 

Yellowish-green. 


Bluish-green. 

Cyan-blue.'^' 

Ultramarine-blue. 


Starting  with  yellow,  we  find  that,  according  to  the  theory 
under  consideration,  it  should  be  produced  by  the  joint 


* Cyan-blue  is  a greenish-blue. 


no 


MODERN  CHROMATICS. 


stimulation  of  the  red  and  green  nerves  ; consequently,  if 
^yc  present  simultaneously  to  the  eye  red  and  green  light, 
the  sensation  produced  ought  to  he  what  we  call  yellow. 
This  can  he  most  perfectly  accomplished  by  mixing  the  red 
and  green  light  of  the  spectrum  ; it  is  possible  in  this  way 
to  produce  a fair  yellow  tint.  The  method  of  rotating 
disks  furnishes,  when  emerald-green  and  vermilion  are  em- 
ployed, a yellow  which  aj)pears  rather  dull  for  two  reasons  : 
first,  because  the  pigments  which  we  call  yellow,  such  as 
clirome-yellow  or  gamboge,  are,  as  will  hereafter  be  shown, 
relatively  more  brilliant  and  luminous  than  any  of  the  red, 
green,  blue,  or  violet  pigments  in  use  ; so  that  these  bright- 
yellow  pigments  stand  in  a se]>arate  class  by  themselves. 
This  circumstance  influences  our  judgment,  ami,  finding  the 
resultant  yellow  far  less  brilliant  than  our  (false)  standard, 
chrome-yellow,  we  are  disap])ointed.  The  second  reason  is, 
that  green  light  stimulates,  as  before  mentioned,  the  violet 
as  well  as  the  green  nerves  ; hence  all  three  sets  of  nerves 
are  set  in  action  to  a noticeable  extent,  and  the  sensation  of 
yellow  is  mingled  with  that  of  white,  and  consequently  is 
less  intense  than  it  otherwise  would  be.  hen  the  green 
and  red  of  the  s])cctrum  are  mingled,  we  have  at  least  not 
to  contend  with  a false  standard,  and  only  the  second  reason 
comes  into  ]day,  and  causes  the  yellow  thus  j)roduced  to 
look  as  though  mingled  with  a certain  quantity  of  white. 
It  was  found  by  the  lamented  J.  d.  Muller  that  green  light 
when  mingled  with  any  other  coloured  light  of  the  spec- 
trum diminished  its  saturation,  and  caused  it  to  look  as 
though  at  the  same  time  some  white  light  had  been  added. 
Idiis  is  what  our  fundamental  diagram  (Fig.  48)  would  lead 
us  to  expect  ; it  is  quite  in  consonance  with  the  theory  of 
Young  and  Helmholtz. 

Having  now  accounted  for  the  fact  that  the  yellow  pro- 
duced by  mixing  red  and  green  light  is  not  particularly 
brilliant,  it  will  be  easy  to  show  how  several  of  the  other 
colour-sensations  are  generated.  Tf,  for  instance,  we  dimin- 


THE  COLOUR  THEORY  OF  YOUNG  AND  HELMHOLTZ.  H7 

ish  the  intensity  of  the  green  light  in  the  experiment  above 
mentioned,  the  resultant  hue  will  change  from  yellow  to 
orange,  red-orange,  orange-red,  and  finally  to  pure  red. 
These  changes  are  best  followed  by  using  the  coloured 
light  of  the  spectrum,  but  may  also  be  traced  by  the  help 
of  Maxwell’s  disks  (Fig.  38),  or  by  the  aid  of  the  glass 
plate  of  Helmholtz  (Fig.  39).  / On  the  other  hand,  if,  in 
the  experiment  now  under  consideration,  the  green  light  be 
.made  to  preponderate,  the  resultant  yellow  hue  will  pass 
into  greenish  yellow,  yellowish  green,  and  finally  green. 
This  accounts  for  the  production  of  more  than  half  the  col- 
our-sensations in  the  list  above  given,  and  the  remaining 
ones,  such  as  ultramarine,  cyan-blue,  and  bluish  green,  can 
be  produced  in  the  same  way  by  mingling  in  proper  pro- 
portions green  and  violet  light,  using  any  of  the  methods 
above  mentioned. 

In  the  cases  thus  far  considered  we  have  presented  to 
the  eye  mixtures  of  two  different  kinds  of  coloured  light, 
or,  to  speak  more  accurately,  two  kinds  of  light  differing  in 
wave-length  ; it  now  remains  for  us  to  account  for  the  pro- 
duction of  colour-sensations  in  those  cases  where  the  eye  is 
acted  on  only  by  one  kind  of  coloured  light,  or  by  light 
having  one  wave-length.  In  the  case  of  red,  green,  or  vio- 
let light,  the  explanation  of  course  lies  on  the  surface  : the 
red  light  stimulates  powerfully  the  red  nerves  and  produces 
the  sensation  we  call  red,  and  so  of  the  others.  But  this 
does  not  quite  exhaust  the  matter  ; for,  according  to  the 
theory  of  Young  and  Helmholtz,  this  same  red  light  also 
acts  to  some  extent  on  the  green  and  violet  nerves,  and  si- 
multaneously produces  to  some  small  degree  the  sensations 
we  call  green  and  violet.  The  result  then,  according  to  the 
theory,  ought  to  be  the  production  of  a strong  red  sensa- 
tion, mingled  with  much  weaker  green  and  violet  sensa- 
tions ; or,  in  other  words,  even  when  the  eye  is  acted  on  by 
the  pure  red  light  of  the  spectrum,  this  red  light  ought  to 
appear  as  though  mingled  with  a little  white  light,  even  if 


118 


MODERN  CHROMATICS. 


none  is  actually  present.  Experiment  confirms  this  theo- 
retical conclusion,  and  here  again  decides  in  favour  of  the 
correctness  of  our  theory.  The  simplest  way  of  making 
the  experiment  would  be  to  temporarily  remove,  were  it 
possible,  the  green  and  violet  nerves  from  a portion  of  the 
retina  of  the  eye,  and  then  to  throw  on  the  whole  retina  the 
pure  red  light  of  the  spectrum.  This  red  light  ought  then 
to  appear  more  intense  and  saturated  when  falling  on  the 
spot  from  which  the  green  and  violet  nerves  had  been  re- 
moved than  when  received  on  the  rest  of  the  retina,  where 
all  three  kinds  of  nerves  were  present.  Now,  though  we 
can  not  actually  remove  the  green  and  violet  nerves  from  a 
spot  in  the  retina,  yet  we  can  by  suitable  means  tire  them 
out,  or  temporarily  exhaust  them,  so  that  they  become  to 
a considerable  extent  insensitive.  If  a small  sj)ot  of  the 
retina  be  exposed  for  a few  moments  to  a mixture  of  green 
and  violet  light  so  combined  as  to  appear  bluish  green,  the 
green  and  violet  nerves  will  actually  become  to  a consider- 
able extent  inoperative  ; and,  when  the  eye  is  suddenly 
turned  to  the  red  of  the  spectrum,  this  spot  of  the  retina 
will,  if  we  may  use  the  expression,  experience  a more 
powerful  and  purer  sensation  of  red  than  the  surrounding 
unfatigued  portions,  Avhere  the  red  will  look  as  if  diluted 
with  a certain  amount  of  white  light.  From  this  experi- 
ment of  Helmholtz  it  appears,  then,  that  it  is  actually  pos- 
sible to  produce  by  artificial  means  colour-sensations  which 
are  more  powerful  than  those  ordinarily  generated  by  the 
light  of  the  spectrum — a point  to  which  we  shall  return  in 
the  following  chapter. 

Having  accounted  now  for  the  production  of  the  colour- 
sensations  red,  green,  and  violet  by  red,  green,  and  violet 
light,  and  noticed  an  interesting  peculiarity  connected  with 
this  matter,  we  pass  on  to  the  others.  Taking  up  the  yel- 
low of  the  spectrum,  we  find  that  it  can  be  produced  by  the 
action  on  the  eye  of  waves  of  light  intermediate  in  length 
between  those  which  give  the  sensations  red  and  green. 


THE  COLOUE  THEORY  OF  YOUNG  AND  HELMHOLTZ.  H9 


These  waves  are  too  short  to  act  very  powerfully  on  the 
red  nerves,  and  too  long  to  set  into  maximum  activity  the 
green  nerves,  but  they  set  both  into  moderate  action  ; the 
result  of  this  joint  action  of  the  two  sets  is  a new  sensation, 
which  we  call  yellow.  Furthermore,  it  may  be  remarked 
that  the  waves  of  the  light  called  yellow  are  far  too  long  to 
produce  any  but  a feeble  effect 'on  the  violet  nerves  ; they 
affect  them  less  than  green  light  does.  From  this  it  results 
that  the  sensation  of  yellow,  when  directly  produced  by 
the  yellow  light  of  the  spectrum,  is  less  mingled  with  that 
of  white,  and  is  purer  than  is  the  case  when  it  is  brought 
about  by  mixing  red  light  with  green  in  the  manner  before 
described.  And  this  explanation  may  serve  to  account  for 
the  fact  that  it  is  impossible,  by  mixtures  of  red  and  green 
light  taken  from  the  spectrum,  to  produce  a yellow  light  as 
pure  and  brilliant  as  the  yellow  of  the  spectrum.  Let  us 
suppose,  in  J:he  next  place,  that,  instead  of  presenting  to 
the  eye  the  yellow  of  the  spectrum,  we  act  on  it  by  the 
light  belonging  to  one  of  the  other  spaces — the  blue,  for 
example.  The  explanation  is  almost  identical  with  that 
just  given  for  the  yellow  : the  waves  constituting  blue  light 
being  too  short  to  powerfully  affect  the  green  nerves,  and 
too  long  to  accomplish  this  with  the  violet  nerves,  both 
green  and  violet  nerves  are  moderately  affected,  giving  the 
sensation  we  call  blue.  Meanwhile  the  blue  light  produces 
very  little  action  on  the  red  nerves,  and  hence  very  little  of 
the  sensation  of  white  is  mingled  with  that  of  blue  ; and 
consequently  this  blue  hue  is  more  saturated  than  when 
produced  by  the  actual  mixture  of  green  and  violet  light. 
In  fact,  J.  J.  Muller  found  that  green  light,  when  mingled 
with  light  from  any  other  part  of  the  spectrum,  produced  a 
hue  which  was  less  saturated  and  more  whitish  than  the 
corresponding  tint  in  the  spectrum  which  the  mixture  imi- 
tated. The  production  of  all  the  other  colour-sensations 
obtained  by  looking  at  the  spectrum  is  explained  in  the 
same  way  by  our  theory.  From  all  this  one  interesting 


120 


MODERN  CHROMATICS. 


conclusion  can  be  drawn,  viz.  : that  there  are  two  distinct 
ways  of  producing  the  same  colour-sensation  ; for  we  have 
seen  that  it  may  be  accomplished  either  by  presenting  to 
the  eye  a mixture  of  green  and  violet  light,  or  simply  one 
kind  of  light,  the  waves  of  which  are  intermediate  in  length 
between  those  of  green  and  violet.  The  eye  is  quite  unable 
to  detect  this  difference  of  origin,  although  a prism  reveals 
it  instantly. 

Having  examined  thus,  with  a degree  of  detail  which 
may  have  seemed  tedious,  the  mode  in  which  colour-sensa- 
tions are  accounted  for  by  the  theory  of  Young  and  Helm- 
holtz, we  pass  to  another  point.  In  order  to  give  more 
exactness  to  this  theory,  it  is  necessary  to  define  with  some 
degree  of  accuracy  the  thi-ee  fundamental  colours  ; for  there 
is  a great  variety  of  reds,  greens,  and  violets.  Helmholtz, 
as  the  result  of  his  first  investigation,  selected  a red  not  far 
from  the  end  of  the  spectrum,  a full  green  and  violet  ; in 
other  words,  the  tints  chosen  were  the  middle  and  end  col- 
ours of  the  spectrum.  iMaxwell,  who  made  a series  of  beau- 
tiful researches  on  points  connected  with  Young’s  theory, 
was  led  to  adopt  as  the  fundamental  colours  a red  which  in 
the  spectrum  lies  between  tlie  fixed  lines  C and  D,  and  is 
distant  from  C just  one  third  of  the  distance  between  C 
and  I).  This  is  a scarlet-red  with  a tint  of  orange,  and  is 
represented  by  some  varieties  of  vermilion.  His  green  is 
situated  between  E and  F,  being  distant  from  E by  one 
quarter  of  the  distance  between  E and  F.  This  colour 
finds  among  pigments  an  approximate  representative  in 
emerald-green.  Instead  of  adopting  a full  violet.  Maxwell 
selected  a violet-blue  midway  between  the  lines  F and  G, 
which  is  represented  tolerably  by  artificial  ultramarine-blue. 
By  subjecting  the  results  of  experiments  on  the  spectrum 
to  calculation,  it  is  possible  to  fix  on  the  position  of  one  of 
the  fundamental  colours,  viz.,  the  green.  Thus  Charles  S. 
Pierce,  using  data  given  in  Maxwell’s  paper,  obtained  for 
this  colour  a slightly  different  result  from  that  just  men- 


THE  COLOUR  THEORY  OF  YOUNG  AND  HELMHOLTZ.  121 

tioned.*  According  to  his  calculations,  the  fundamental 
green  has  a wave-length  of  524  ten-millionths  of  a milli- 
metre, and  is  situated  between  the  lines  E and  h,  being  one 
third  of  the  distance  E h from  E,  whereas  Maxwell’s  green 
is  just  beyond  h.  J.  J.  Muller,  who  conducted  an  impor- 
tant investigation  on  this  subject  by  a quite  different  meth- 
od, arrived  at  a somewhat  dilfei;ent  result  for  the  position 
of  the  green,  and  assigned  to  it  a wave-length  of  506 '3  ten- 
millionths  of  a millimetre.  This  position  in  the  spectrum 
is  nearer  the  blue  than  the  positions  given  by  Maxwell  and 
Pierce,  and  the  tint  is  more  of  a bluish  green.  Again,  von 
Bezold,  basing  his  calculations  on  the  experimental  results 
of  Helmholtz  and  J.  J.  Muller,  reached  a conclusion  not 
differing  much  from  those  of  Maxwell  or  Pierce.  He  se- 
lects a green  in  the  middle  of  the  normal  spectrum  between 
E and  5,  but  nearer  h.  INone  of  these  results  differ  very 
greatly  ; in  fact,  the  differences  can  hardly  be  well  indicated 
in  a spectrum  of  the  size  of  this  page.  All  these  greens 
may  be  imitated  by  using  the  pigment  known  as  emerald- 
green,  alone  or  mixed  either  with  a small  quantity  of 
chrome-yellow  or  cobalt-blue.  Hence  all  these  green  hues 
are  of  the  most  powerful  or,  as  artists  would  say,  over- 
powering character. 

The  exact  determination  of  the  other  two  fundamental 
colours  is  a more  difficult  matter,  so  that  even  the  advocates 
of  Young’s  theory  have  not  entirely  agreed  among  them- 
selves upon  the  exact  colours.  Maxwell  taking  ultramarine- 
blue,  Helmholtz  and  J.  J.  Muller  violet,  as  the  third  funda- 
mental. These  fundamental  colours  are  among  the  most 
saturated  and  intense  of  those  furnished  by  the  spectrum. 
Compared  with  them,  the  blue  of  the  spectrum  is  a feeble 
tint,  so  that  it  has  often  been  remarked  by  Rutherfurd  that, 
in  comparison  with  the  other  colours,  it  appears  of  a slaty 
hue.  The  greenish  yellow  is  also  feeble  ; and,  as  is  Avell 


* “ Proceedings  of  the  American  Academy  of  Arts  and  Sciences,  1873.” 


12:2 


MODERN  CHROMATICS. 


known,  pure  yellow  is  found  in  the  spectrum  in  very  small 
quantity  and  of  no  great  intensity.  The  orange-yellow  is 
also  much  weaker  than  the  red,  and  the  orange  only  be- 
comes strong  as  it  approaches  redness  in  hue.  From  this  it 
very  naturally  follows  that,  if  a normal  spectrum  is  cast  on 
a white  wall  in  a room  not  carefully  darkened,  scarcely 
more  than  .the  three  fundamental  colours  will  be  discerned, 
red,  green,  and  blue-violet  ; the  other  tints  can  with  some 
difficulty  be  made  out,  but  at  first  sight  they  strike  the  un- 
])rejudiced  observer  simply  as  the  places  where  the  three 
})rincipal  colours  blend  together.  The  representatives  of 
the  fundamentals  among  pigments  are  also  those  which 
surpass  all  others  in  strength  and  saturation.  One  of  the 
fundamental  colours,  red,  is  used  without  much  difficulty  in 
])ainting  and  decoration  ; the  other  two  are  more  difficult 
to  manage,  particularly  the  green.  The  last  colour,  even 
when  subdued,  is  troublesome  to  handle  in  painting,  and 
many  artists  avoid  it  as  far  as  possible,  or  admit  it  into 
their  work  only  in  the  form  of  low  olive-greens  of  various 
shades.  When  the  tint  approaches  the  fundamental  green, 
and  is  at  the  same  time  bright,  it  becomes  at  once  harsh 
and  brilliant,  and  the  eye  is  instantly  arrested  by  it  in  a 
disagreeable  manner. 


NOTE  TO  CTTAPTER  IX. 

Yovno  does  not  appear  to  be  the  first  who  proposed  red,  green, 
and  violet  as  the  three  fundamental  colours.  As  far  back  as  1792 
Wiinsch  was  led  to  the  same  result  by  his  experiments  on  mixtures 
of  the  coloured  ravs  of  the  spectrum.  The  title  of  his  work  is 
“Yersuche  und  Beobachtungen  iiber  die  Farben  des  Lichtes” 
(Leipsic,  1792).  An  abstract  of  the  contents  is  contained  in  the 
“ Annales  de  Chimie,”  LXIY.,  135. 

A.  M.  flayer  has  recently  called  attention  to  the  way  in  which 
Younff  was  led  to  adopt  red,  green,  and  violet  as  the  three  funda- 
mental colours,  and  has  shown  that  A oung  at  first  “ selected  red, 


NOTE  TO  CHAPTER  IX. 


123 


yellow,  and  blue  as  the  three  simple  colour-sensations ; second,  that 
he  subsequently  modified  his  hypothesis,  and  adopted  red,  green, 
and  violet  as  the  three  elementary  colour-sensations,  showing  that 
up  to  the  date  of  this  change  of  opinion  all  of  his  ideas  on  the  sub- 
ject were  hypothetical,  and  not  based  on  any  observations  of  his 
own  or  others;  third,  that  this  change  of  opinion  as  to  the  three 
elementary  colours  was  made  on  the  basis  of  a misconception  by 
Wollaston  of  the  nature  of  his  celebrated  observation  of  the  dark 
lines  in  the  solar  spectrum,  and  also  on  the  basis  of  an  erroneous 
observation  made  by  Young  in  repeating  Wollaston’s  experiment; 
fourth,  that  Young  subsequently  tested  his  hypothesis  of  colour- 
sensation,  and  found  that  it  was  in  accord  with  facts  reached  by 
experiment,  and  that  these  experiments  then  vindicated  his  hy- 
pothesis and  raised  it  to  the  dignity  of  a theory.”  (“American 
Journal  of  Science  and  Arts,”  April,  1876.) 

Fig.  43  (page  114)  shows  the  intensities  of  the  three  primary  sensa- 
tions, red,  green,  and  violet,  as  estimated  by  Helmholtz.  The  intensi- 
ties were  afterward  measured  by  Maxwell,  and  found  to  differ  slightly 
in  the  case  of  different  eyes.  In  Fig.  44  the  letters  0 D E F G de- 
note the  fixed  lines  of  the  solar  spectrum ; the  curve  R R R,  the 


C D ^ E F G 


Fig.  44.— Curves  showing  the  Intensity  of  the  Fundamentel  Sensations  in  Different 
Parts  of  the  Solar  Spectrum.  (Maxwell.) 

intensity  of  the  sensation  of  red  in  different  parts  of  the  spectrum ; 
GGG  is  the  curve  for  the  green,  and  B BB  that  for  the  violet-blue 
sensation.  Fig.  31  (page  104)  shows  the  same  curves  as  obtained  by 
another  observer.  (“  Philosophical  Transactions  ” for  18G0,  vol.  cl.) 


CHAPTER  X. 


ON  THE  MIXTURE  OF  COLOURS. 

Those  who  watch  a painter  at  work  are  astonished  at 
the  vast  number  and  variety  of  tints  which  can  be  made  by 
mixing  in  varying  proportions  a very  few  pigments  : from 
red  and  yellow  there  is  produced  a great  series  of  orange 
tints  ; yellow  and  blue  furnish  a multitude  of  green  hues  ; 
blue  and  red,  a series  of  purples.  The  results  seem  almost 
magical,  and  we  justly  admire  the  skill  and  knowledge 
which  enable  him  in  a few  seconds  accurately  to  match  any 
colour  which  is  within  the  compass  of  his  palette.  As  we 
continue  our  observations  we  soon  find  that  the  matter  is 
more  complicated  than  it  appeared  at  first  sight,  each  pig- 
ment having  a particular  set  of  properties  which  it  carries 
into  its  mixtures,  and  these  properties  being  by  no  means 
fully  indicated  by  its  mere  colour.  Thus,  some  blue  pig- 
ments furnish  fine  sets  of  greens,  while  others,  as  beautiful 
and  intense,  yield  only  dull  olive-greens  ; some  reds  give 
glowing  purples,  while  from  others  not  less  bright  it  is  pos- 
sible to  obtain  only  dull,  slaty  purples.  Before  touching 
on  these  com])licated  cases  it  will  be  well  to  study  this  sub- 
ject under  its  simplest  aspects,  and  to  content  ourselves  for 
the  present  with  making  an  examination  of  the  effects 
which  are  produced  by  mixing  light  of  different  colours. 
This  can  not  be  brought  about  by  mixing  pigments,  as  was 
for  a long  time  supposed.  In  some  cases  the  mixture  of 
pigments  gives  results  more  or  less  like  those  produced  by 


ON  THE  MIXTURE  OF  COLOURS. 


125 


the  mixture  of  coloured  light,  but  as  a general  thing  they 
differ,  and  in  some  cases  the  difference  is  enormous.  In 
the  previous  chapter,  for  instance,  it  was  shown  that  while 
the  mixture  of  yellow  with  blue  pigments  produced  inva- 
riably a green  hue  of  varying  intensity,  the  mixture  of  blue 
with  yellow  light  gave  a more  or  less  pure  white,  but  under 
no  circumstances  anything  approaching  green.  The  mix- 
ture of  two  masses  of  coloured  light  can  easily  be  effected 
in  a simple  manner  so  as  to  be  exhibited  readily  to  a large 
audience.  Two  magic  lanterns  are  to  be  employed,  as 
shown  in  Fig.  45,  the  usual  slides  being  replaced  by  plates 


Fig.  45.— Two  Magic  Lanterns  casting  Yellow  and  Blue  Light  on  the  same  Screen, 
where  it  forms  White. 


of  coloured  glass,  as  indicated.  Each  lantern  then  will  fur- 
nish a large  bright  circle  of  coloured  light,  which  can  be 
projected  on  a white  screen,  the  room  in  which  the  experi- 
ments are  made  being  first  darkened.  In  this  way  it  will 
be  found  that  violet-blue  light  when  mixed  with  green 
light  gives  blue  or  greenish-blue  light,  according  to  the 
proportions  of  the  two  constituents  ; green  with  red  gives 
various  hues  of  orange  or  whitish  yellow,  instead  of  a set 
of  dull,  indescribable  shades  of  greenish,  reddish,  or  brown- 
ish grey,  as  is  the  case  with  pigments.  These  and  many 


1:26 


modern  chromatics. 


other  beautiful  experiments  on  the  mixture  of  coloured 
light  can  easily  he  made  ; and  even  the  effects  produced  by 
varyino-  the  intensity  of  either  of  the  masses  of  coloured 
lio-ht  c"an  readily  be  studied  by  gradually  diminishing  the 
brightness  of  one  of  the  coloured  circles,  the  other  leinain- 

ino*  constant.  ^ ^ 

'I’o  all  these  experiments  it  may  be  objected  that  we  aie 
not  using  coloured  light  of  sufficient  purity  - that  our  yel- 
low glass  transmits,  as  was  shown  in  Chapter  ' H-.  not  on  ) 
yellow  but  red,  orange,  and  green  light  ; and  that  the  otl  e 
stained  glasses  are  not  much  better  off  m this  respect, 
lienee,  to  meet  all  such  objections,  physicists  have  been 
liiially  obliged  to  conduct  their  researches 
on  the  pure  coloured  rays  of  the  spectrum.  1 he  difficulties 
eucouutcrcd  in  the  use  of  this  method  are  much  gieatei, 
but  the  results  so  obtained  are  far  more  precious.  ^ ery 
beautiful  investigations  of  this  character  have  been  made 
by  Hel.iil.olt.,  Maxwell,  and  .T.  J.  Muller  The  general 
character  of  their  results  is  something  like  this  . by  inixm 
two  kinds  of  pure  coloured  light  they  obtained,  as  a geneia 
thiim,  light  having  a colour  different  from  either  of  the 
ori.rinaringrcdients  ; for  example,  red  and  yellowish  green 
.vave  an  orange  hue  which  looked  in  all  respects  nke  the 
pure  orange  of  the  spectrum  ; also,  in  this  new  ' 

was  impossible  by  the  ci/e  to  detect  the  presence  of  eithei 
red  or  vellowish-grecn  light.  This  was  true  of  =>’1 
in  no  case  could  the  original  ingredients  be  detected  bj  the. 
eve  In  this  respect  the  eye  differs  from  the  ear  , oi  y 
inactice  it  is  possible  with  the  unaided  ear  to  analyze  up 
compound  sounds  into  the 

at  liast  to  some  extent.  It  was  also  ascertained  that  the 
same  colour  could  be  produced  in  several  different  ways 
that  is,  by  combining  together  different  pairs  of  spect 
colours.  Thus,  violet  with  cyan-blue  gave  an 
hue  but  violet  gave  the  same  colour  nhen  mixet 
bluish-green,  or  e\-on  with  green  ; in  the  last  case  the  tint 


ON  THE-  MIXTURE  OF  COLOURS. 


127 


was  somewhat  whitish.  By  mixing  certain  colours  of  the 
spectrum,  it  was  found  that  one 


new  colour  or  colour-sensation 
could  he  produced  which  origi-  “ 
nally  was  not  furnished  by  the  ® 
plain  pure  spectrum  itself  ; we  re-  c 
fer  to  purple,  or  rather  to  the 
whole  class  of  purples,  ranging 
from  violet-purple  to  red-purple. 
These  are  produced  by  mixing  the 
end  colours  of  the  spectrum,  red 
and  violet,  in  varying  proportions. 
Furthermore,  mixtures  of  certain 
colours  of  the  spectrum  gave  rise  ^ 
to  white  ; this  was  true,  for  ex-  ^ 
ample,  of  red  and  bluish-green, 
and  of  yellow  and  ultramarine- 
blue.  The  white  in  these  two  f 
cases,  though  so  different  in  origin, 
had  exactly  the  same  appearance 
to  the  eye.  Again,  by  mixing 
three  or  more  spectral  colours  no 
new  hues  were  produced,  but  sim- 
ply varieties  of  those  which  could 
be  obtained  from  two  colours. 

Such  is  the  general  character  g 
of  the  results  obtained  by  mixing 
together  masses  of  pure  coloured 
light  ; we  propose  now  to  enter  a 
little  more  into  detail  respecting 
this  very  interesting  matter,  and 
to  examine  the  laws  which  control 
the  production  of  the  resultant 
hues. 

It  was  ascertained  by  Muller, 
who  worked  under  the  direction 


■ Eed-orange. 

• Orange. 

• Orange-yellow. 
Y ellow. 

I Greenish-yellow 
> and 
I Y ello wish-green . 


Green  and 
Blue-green. 


> 


.Red. 


Cyan-blue. 


y Blue  and 
/ Blue-violet. 


> Violet. 


Fig.  4G.— Prismatic  Spectrum. 


^23  MODERN  CHROMATICS. 

of  Helmholtz,  that  all  the  colours  of  the  spectrum,  Fig.  46, 
from  red  to  yellowish-green,  gave  by  mixture  resultant 
hues  which  were  always  identical  with  some  of  the  colours 
situated  between  red  and  yellowish-green,  thus  . 

Table  1. 

Ked  and  yellowish-green  gave Orange  or  yellow  * 

Red  and  yellow  gave 

Orange  and  yellowish-green  gave \ellow. 

'Phe  effect  of  tl.e  mixture  in  tliese  cases  was  to  produce 
colours  which  were,  to  all  appearance,  as  pure  as  the  cor- 
respondimr  colours  of  the  si.ectrum  itself. 

l.’urthtT.nore,  all  colours  of  the  spectrum  from  violet  to 
hluish-green  furnished  mixtures  corresponding  to  the  col- 
ours contained  between  these  limits,  thus  : 

Table  II. 

Rluish-"reen' and  ultramarine-blue  gave. ..  Cyan-blue.  • u,  « * 

*^‘ui^n  „retu  Cvan-blue  or  ultraraanne-blue.* 

Bluish-green  and  violet  ga\e i ^ 

Violet  and  cyan-blue  gave ramarin 

in  these  cases  also  the  resulting  tints  could  not  be  distin- 
miished  from  the  corresponding  spectral  colours  The  re- 
sults thus  far  are  simple  in  character,  and  easily  remem- 
bered by  any  one  who  recollects  the  arrangement  of  the 

colours  of  the  spectrum.  . 

On  the  other  hand,  r/ree>i,  when  mixed  wi  h any  colour 
of  the  spectrum,  gave  a resultant  colour,  vvhich  was  less 
saturated  or  intense,  and  appeared  more  whitish,  than 
corresponding  spectral  tint,  thus  . 

Table  III. 

f Orange,  ^ 

) VpIIow  > whitish. 

Green  and  red  gave j Yellow'ish-green,  i 


* According  to  the  proportions. 


ON  THE  MIXTURE  OF  COLOURS. 


129 


Green  and  yellow  gave Yellowish-green — whitish. 

Green  and  cyan-blue  gave Bluish-green — whitish. 

r Ultramarine-blue,  ^ 

Green  and  violet  gave ■<  Cyan-blue,  > whitish. 

( Bluish-green,  ) 

Yellowish-green  and  bluish-green  gave. Green — very  whitish. 

Mtiller  made  a careful  determination  of  the  position  in  the 
spectrum  of  the  green  which  had  the  greatest  effect  in 
diminishing  the  saturation,  and  consequently  was  most  in- 
fluential in  generating  pale  or  whitish  tints.  It  was  situ- 
ated between  the  fixed  lines  b and  F,  at  one  third  the  dis- 
tance between  b and  F,  measured  from  b.  This  colour  is  a 
bluish-green,  and  can  be  imitated  by  mixing  emerald-green 
with  a small  quantity  of  cobalt-blue.  According  to  Mtiller, 
as  already  stated,  this  is  the  fundamental  green  : its  wave- 
length is  506*3  ten-millionths  of  a millimetre. 

Having  considered  the  effects  produced  by  mixing  the 
colours  of  the  spectrum  situated  on  either  side  of  the  green, 
and  also  the  effects  produced  by  green  itself  in  mixture,  it 
remains  to  examine  the  mixtures  of  the  colours  located 
right  and  left  of  green,  thus  : 

Table  IV. 

Red  and  ultramarine-blue  gave. . . Violet — slightly  whitish. 

Red  and  cyan-blue  gave Ultramarine  or  violet — whitish. 

Orange  and  violet  gave Red — whitish. 

Red  and  violet  gave Purple — whitish. 

Orange  and  ultramarine  gave.  . . . Purple — whitish. 

These  results  may  at  first  sight  not  seem  as  simple  and  ob- 
vious as  those  mentioned  above,  but,  when  the  arrangement 
of  the  colour-diagram*  has  been  explained,  it  will  be  seen 
that  they  are  strictly  analogous  to  the  cases  before  given. 

It  may  have  been  noticed  by  the  reader  that  the  series 
of  pairs  given  in  the  tables  thus  far  do  not  entirely  exhaust 


* Sec  Chapter  XIV. 


modern  chromatics. 

ioO 

rproduction  of  coloured  but  of  whrte  hgbt,  thus  . ■ 

Table  V. 

White. 

Red  and  bluish-green^  gave White. 

Orange  and  cyan-blue  gave White. 

Yellow  and  ultramarine  gave White. 

Greenish-yellow  and  violet  gave 

.!  turi,  “5,  J.m.«a,y  col.u,  in 

the  colour  called  1>«''P^’  understood,  have 

TU„,  «1»™«  ^ ,„,„„«i„n, 

;;s“nS 

,nre,  of  ™ ";V”“»l'in  nmwding  ■«>"? 

With  maten.dwhic  , ^r  art,  which  otherwise 

our-problcms,  prcseutei  } • experiments  them- 

would  be  <imte  bet  oml  oi  ‘ foj.  their  proper 

selves  unfortunately  d _ ’ much  pa- 

execution  require  knowledge  am  sktU 

tionce.  There  is,  however,anothermethmlotm.^^ 

light  to  which  no  such  ht’terested  in  this  sub- 

quite  within  the  teach  o . tatino-  disks  which  has 

iect.  We  refer  to  the  method  of  card- 

already  once  or  twice  been  ^qth  vermilion 

board  be  painted,  .s  ^ th!n  set  i^  rapid  rotation, 

and  a bluish-green  ^ ^f  the  observer), 

these  colours  will  be  mix  ( uniform  tint, 

and  the  whole  ^rmixture  of  the  coloured  light 

which  will  he  that  due  o c - YVhen 

sent  out  from  the  two  halves  of  the  disk  (T  i^ 

■ 1 See  Chapter  IX. 

* Or  rather  green-blue.  ' 


ox  TUE  MIXTURE  OF  COLOURS. 


131 


we  analyze  this  experiment,  we  find  that  what  actually  does 
take  place  is  this  : At  any  one  particular  instant  a certain 
portion  of  the  retina  of  the  eye  will  he  affected  with  red 
light ; the  disk  then  turns  and  presents  to  the  same  portion 
of  the  retina  bluish-green  light  ; then  follows  red,  then 
bluish-green,  etc.  Hence  the  retina  is  really  acted  on  by 
alternate  presentations  of  the  fwo  masses  of  coloured  light,- 
the  intervals  between  these  substitutions  being  something 
less  than  one  fiftieth  of  a second,  l^ow,  it  so  happens  that 
these  alternate  presentations  have  the  same  effect  on  the 
eye  as  simultaneous  presentations.  This  is  not  the  least 
valuable  result  of  the  spectral  experiKLents  above  described, 


Fig.  47.— Disk  painted  with  Ver- 
milion and  Blue -green. 


Fig.  48.— Appearance  presented 
by  Eed  and  Blue-green  Disk 
when  in  Kapid  Rotation. 


for  it  enables  us  by  an  easy  method  to  pursue  our  colour- 
investigations  without  having  direct  recourse  to  the  spec- 
trum. There  is  one  respect  in  which  the  mixture  by  rotat- 
ing disks  actually  does  differ  from  that  where  the  presenta- 
tion is  simultaneous.  If  we  simultaneously  present  to  the 
eye  two  masses  of  coloured  light,  it  is  plain  that  the  lumi- 
nosity of  the  mixture  will  be  equal  to  the  sum  of  the  lumi- 
nosities of  the  two  components  (or  at  least  must  approximate 
to  it)  ; thus,  if  the  luminosity  of  our  red  light  be  25,  and 
that  of  our  greenish-blue  30,  the  luminosity  of  the  tint  of 
mixture  will  be  55.  If,  however,  these  two  masses  of  light 
act  on  the  eye  alternately,  as  is  the  case  with  rotating 


132 


MODERN  CHROMATICS. 


disks,  the  luminosity  of  the  mixture-tint  will  be  not  the 
sum  but  the  mean  of  the  separate  luminosities  ; that  is,  27 J.* 
This  method  of  mixing  colours  was  mentioned  in  the 
second  century,  in  the  “ Optics  ” of  Ptolemy.f  It  was 


rediscovered  by  Musschenbroek  in  1702,  and  finally  greatly 
imj)roved  by  Maxwell.  The  last-named  physicist  modified 
the  disks,  so  that  it  became  j)0ssible  easily  to  mix  the  col- 
ours in  any  desired  pro})ortion.  This  important  improve- 


ment is  effected  simply  by  cutting  a slit  through  the  disk 
from  the  centre  to  the  circumference,  as  indicated  in  Fig. 
40.  The  slit  enables  the  experimenter  to  combine  two  or 
even  more  disks  on  the  same  axis,  and  to  adjust  them  so 

* Compare  the  results  obtained  by  the  author  and  given  in  Chapter  III. 
f “ Bibliographie  Aualytique,”  by  J.  Plateau  (1877). 


ON  THE  MIXTURE  OF  COLOURS. 


133 


that  they  present  their  respective  surfaces  in  any  desired 
proportion.  (See  Figs.  50  and  51.)  The  relative  propor- 
tions of  the  two  colours  can  then  be  obtained,  as  was  done 
by  Maxwell,  by  placing  a graduated  circle  around  the 
disks.  The  author  finds  it  better  to  apply  a graduated  cir- 
cle of  pasteboard  to  the  face  of  the  disk,  this  circle  being 
made  a little  smaller  than  the  disk  itself  ; the  centring  is 
insured  by  its  contact  with  the  axis  on  which  the  disk  is 
fastened.  (See  Fig.  52.)  Maxwell  divided  his  circle  into 


100  instead  of  360  parts  ; this  is  convenient,  and  tenths  of 
a division  can  readily  be  estimated  by  the  eye.  There  is 
another  very  important  feature  connected  with  Maxwell’s 
disks  : they  can  easily  be  arranged  so  as  to  furnish  colour- 
equations,  which  are  of  great  use  in  chromatic  studies. 
For  example,  returning  to  our  compound  disk  of  red  and 
bluish-green,  and  remembering  that  these  colours  are  com- 
plementary, it  is  evident  that,  if  we  give  to  the  red  and 
bluish-green  surfaces  the  proper  proportions,  we  can  from 
them  produce  white,  or,  what  is  the  same  thing,  a pure 
grey.  But  a pure  grey  can  also  be  produced  by  rotating  a 
white  and  black  disk  similarly  arranged.  Hence,  in  an  ex- 
periment of  this  kind,  we  place  on  the  axis,  first,  the  disks 
of  vermilion  and  bluish-green,  and  then  on  the  same  axis 
smaller  disks  of  black  and  white  pasteboard,  cut  in  a simi- 
lar manner  with  radial  slits.  By  repeated  trials  we  can 


Fig.  52.— Mode  of  measuring  the  Colours  on  the  Disks. 


134 


MODERN  CHROMATICS. 


arrange  the  coloured  disks  so  that  they  furnish  a grey  as 
neutral  and  pure  as  that  due  to  the  black  and  white  disks 


Fio. 


W. 


V Ml  this  can  be  put  in  the  form  of  an  equation 

tonii  gre\.  Ail  U\u  <-*  i oi  ^vhitc  + 7B•T  black, 

bv  writing  :ib  red  + r,4  blue-green  = A 3 ^^mte^-  ^ 

white  paper  ft  is  not  .tr 

on  this  pom  , s ^ r^aintinf^  its  surface  with  lamp- 

pasteboard  IS  prepared  by  pamtin^ 


ON  THE  MIXTURE  OF  COLOURS. 


135 


black  in  powder  to  which  just  enough  spirit  varnish  has 
been  added  to  cause  it  to  adhere  closely,  but  not  to  shine, 
then  a uniform  surface  will  be  obtained,  which  reflects  a 
small  but  definite  amount  of  the  light  falling  on  it.  If,  as 
before,  we  set  the  luminosity  of  white  pasteboard  as  100, 
then  that  of  this  kind  of  black  paper  will  be  5‘2  ; or,  in 
other  words,  it  reflects  about  five  per  cent,  as  much  light 
as  white  pasteboard.  This  knowledge  enables  us  to  correct 
the  eguation  just  given  ; instead  of  21 ’3  white,  we  should 
write  25*4. 

In  the  above  example  we  have  taken  the  case  of  two 
complementary  colours,  and  have  obtained  a measure  for 
the  white  or  grey  light  which  they  furnish  by  mixture  ; 


Fig.  54.— Vermilion  and  Emerald-green  Disks  arranged  to  produce  a Yellow  by  nota- 
tion. This  yellow  is  imitated  by  small  chrome-yellow,  black,  and  white  disks,  as 
arranged  in  the  figure. 

when  the  resultant  tint  is  not  grey,  but  some  decided  col- 
our, we  can  in  a similar  way  assign  to  it  a numerical  value. 
Take  the  case  of  vermilion  and  emerald-green : Disks 
painted  with  these  colours  can  be  made  to  furnish  a whitish 
yellow,  as  demanded  by  Young’s  theory,  and  we  can  ex- 
press the  value  of  this  yellow  in  terms  of  chrome-yellow ; 
that  is,  by  darkening  chrome -yellow  and  rendering  it  pale. 
This  we  accomplish  by  combining  chrome-yellow  with  a 
black  and  a white  disk.  As  the  result  of  an  experiment 
of  this  kind,  the  author  obtained  the  equation  ; Yerm. 
51  -f  em.-green  40  = ch.-yel.  20  -p  white  8 + black  72. 


136 


MODERN  CHROMATICS. 


The  compound  disk  properly  arranged  is  seen  in  Fig.  54. 
d'he  reader  may  be  somewhat  surprised  to  notice  that  it 
was  necessary  to  dull  the  chrome-yellow  so  greatly  before 
making  it  similar  in  colour  to  the  yellow  produced  by  mix- 
ing the  green  and  red  light  ; it  must,  however,  be  remem- 
bered that  chrome-yellow  does  not  quite  belong  to  the  same 
set  of  colours  of  which  vermilion  and  emerald-green  are 
members  ; that  is  to  say,  if  we  represented  the  red  space  of 
a normal  spectrum  with  vermilion,  and  the  green  space 
with  emerald-green,  then  chrome-yellow  would  be  too 
bright  or  luminous  for  the  yellow  space,  and  we  should 
have  to  su])stitute  for  it  a less  brilliant  yellow  pigment. 

In  the  same  manner,  with  the  aid  of  properly  painted 
disks,  we  can  make  a series  of  experiments  on  the  mixture 
of  other  colours,  and  satisfy  ourselves  of  the  correctness  of 
the  results  already  given  in  this  chapter.  For  instance,  by 
combining  a yellow' witli  a vermilion  disk  in  various  pro- 
portions, we  obtain  a series  of  orange  or  orange-yellow  hues, 
which  are  as  saturated  in  appearance  as  the  original  con- 
stituents. Red  lead  with  a yellowish-green  disk  gave  a 
fair  yellow,  and  the  same  yellowish-green  disk  when  com- 
bined with  vermilion  furnished  a fine  orange  or  yellow, 
according  to  the  proportions.  Tliese  correspond  to  the  re- 
sults given  in  Table  I.  In  the  same  way  those  contained 
in  the  other  tables  can  be  verified.  Xaturally,  some  care 
must  be  exercised  in  the  selection  of  the  pigments  with 
which  the  disks  are  painted  ; thus,  the  author  finds  that  the 
])ure  red  of  the  S])cctrum  can  be  imitated  by  vermilion 
washed  over  with  carmine.  Vermilion  itself  corresponds 
to  the  red  space  of  the  spectrum  about  half  way  between 
C and  D ; red  lead  answers  for  a red-orange  situated  nearer 
still  to  D,  etc.  The  parts  of  the  spectrum  which  these  and 
other  pigments  represent  are  indicated  in  Chapter  III.,  to 
which  the  reader  is  referred  for  further  information. 

In  preparing  a set  of  disks  for  accurate  experiments,  it 
will  be  necessary  of  course  to  compare  their  colours  care- 


ON  THE  MIXTURE  OF  COLOURS. 


137 


fully  with  those  regions  of  the  spectrum  which  they  are 
intended  to  represent.  This  can  be  done  with  the  aid  of 
the  spectroscope  in  the  method  indicated  in  Chapter  III. 
A set  of  disks  with  carefully  determined  colours  is  quite 
valuable,  not  only  for  experiments  of  this  kind,  but  also  to 
enable  us  to  produce  a vast  variety  of  tints  at  will,  which 
can  be  recorded  and  afterward  accurately  reproduced  when 
necessary. 

We  pass  on  now  to  the  description  of  a beautiful  and 
simple  piece  of  apparatus  contrived  by  Dove,  for  mixing 
the  coloured  light  furnished  by  stained  glass,  and  called  by 
him  a dichrooscope.  This  consists  of  a box  81  millimetres 
long,  75  high,  and  70  broad  ; three  sides  are  open,  but  can 
be  closed  by  opaque  slides  or  by  plates  of  coloured  glass. 
(See  Fig.  55,  which  shows  the  box  in  persj^ective.)  Fig.  56 


is  a vertical  section,  in  which  G R and  R D are  plates  of 
coloured  glass  ; G P is  an  opaque  slide  made  of  blackened 
cardboard,  in  which  a square  aperture  has  been  cut  ; P R 
represents  a set  of  six  glass  plates  made  from  window- 
glass  of  the  best  quality  ; these  are  of  course  colourless. 
At  S S,  Fig.  57,  is  a silvered  mirror,  and  at  N a Nicol’s 
prism.  The  action  of  the  apparatus  is  as  follows  : Let  us 
suppose  that  G R is  a plate  of  green  glass,  and  R D one  of 
red  ; then  the  light  from  the  sky,  striking  on  the  mirror 
S S,  is  reflected  through  R D and  the  plates  P R,  and 


Fig.  55. — The  Box  of  Dove’s  Dichrooscope. 
The  plates  of  coloured  glass  are  removed 
and  the  three  sides  left  open.  The  six 
plates  of  window-glass  are  shown. 


Fig.  56.— The  Dichrooscope  shown 
in  section. 


MODERN  CHROMATICS. 


138 

finally  reaches  the  eye  ; it  will  of  course  be  coloured  red. 
Hut  light  from  the  sky  also  falls  on  the  plate  of  green  glass, 
(i  K penetrates  it,  is  reflected  from  the  glass  plates  at  P R, 
and  also  reaches  the  eye.  The  eye,  then,  will  be  simulta- 
neouslv  acted  on  by  red  and  green  light ; and,  if  the  Mc- 
ol’s  prism  at  X be  removed,  this  mixture  will  be  seen,  but 
we  shall  have  no  means  of  regulating  the  proportions  of  the 
re<l  and  green  light.  Hut,  by  restoring  the  Xicol’s  prism 
to  its  place,  and  rotating' it,  it  is  possible  to  mix  the  red  and 
green  light  in  any  desired  proportion.*  When  the  app.v 


ratiis  is  iirovidcd  with  red  and  green  glass  as  above  indi- 
cated a dull  yellow  will  sometimes  be  given  without  the 
use  of  the  Xicol’s  prism  ; with  its  aid,  this  can  always  be 
accomplished  ; the  yellow  will  pass  into  greenish-yellow  or 
orange,  according  as  the  proportions  of  the  two  constitu- 
ents ^are  varied."  It  is  best  to  employ  glasses  the  tint  of 
which  is  not  too  dark,  as  we  do  not  readily  recognize  dark 
yellow  as  such.  The  author  easily  obtained  pieces  of  green 

* Many  beautiful  experiments  witb  polarized  iight  can  be  made  with 
this  little  apparatus ; for  an  account  of  them  the  reader,  is  referred  to  Pog- 
gendoi-rs  “ .tnnalen,”  ex,,  p.  265,  or  to  the  “ American  Journal  of  ..cience, 
vol.  xxxi.,  January,  1861.’ 


ON  THE  MIXTURE  OF  COLOURS. 


139 


ft 

and  purple  glass  which  gave  pure  white  ; yellow  and  blue 
glass  also  did  the  same.  If  the  hue  of  the  yellow  glass  was 
too  deep,  the  white  was  always  tinged  pinkish.  Red  and 
yellow  gave  orange  ; green  and  yellow,  yellowish-green  ; 
red  and  blue,  purple.  All  these  results  are  quite  in  accord- 
ance with  those  obtained  by  mixing  the  coloured  light  of 
the  spectrum.  ' 

In  the  previous  chapter  we  have  described  a method 
which  was  contrived  long  ago  by  Lambert  for  mixing  the 
coloured  light  from  painted  surfaces  (see  Fig.  39,  page  110). 
The  light  from  the  blue  paper  is  transmitted  directly  to  the 
eye,  and  that  from  the  yellow  paper  reaches  the  eye  after  re- 
flection ; their  action  is  of  course  simultaneous.  By  moving 
the  two  pieces  of  paper  nearer  together  or  farther  apart,  it 
is  possible  to  vary  their  apparent  brightness,  and  thus  to 
regulate  the  proportion  of  blue  and  yellow  light  which 
reaches  the  eye  ; the  yellow  will  predominate  when  the 
papers  are  near  together,  the  blue  as  they  are  moved  fur- 
ther apart.  Chrome-yellow  (the  pale  variety)  and  ultra- 
marine-blue, when  combined  in  this  apparatus,  give  an 
excellent  white,  and  emerald-green  and  vermilion  give  a 
yellowish  or  orange  tint,  according  to  the  arrangement.  It 
is  difficult  to  obtain  a good  representative  of  violet  among 
the  pigments  in  use  by  artists  ; the  author  finds  that  some 
samples  of  the  aniline  colour  known  as  “ Hoffmann’s  violet 
B B ” answer  better  than  any  of  the  ordinary  pigments. 
If  a deep  tint  of  its  alcoholic  solution  be  spread  over  paper, 
and  combined  in  the  instrument  with  emerald-green,  a blue, 
a greenish-blue,  or  a violet -blue  can  readily  be  produced. 
It  is  evident  that  a multitude  of  experiments  of  this  char- 
acter can  be  made,  the  number  of  colours  united  at  one 
time  being  limited  to  two.  The  results  of  course  agree 
with  Young’s  theory. 

Another  method  of  mixing  coloured  light  seems  to  have 
been  first  definitely  contrived  by  Mile  in  1839,  though  it 
had  been  in  practical  use  by  artists  a long  time  previously. 


140 


MODERN  CHROMATICS. 


We  refer  to  the  custom  of  placing  a quantity  of  small  clots 
of  two  colours  very  near  each  other,  and  allowing  them  to 
be  blended  by  the  eye  placed  at  the  proper  distance.  Mile 
traced  tine  lines  of  colour  parallel  to  each  other,  the  tints 
being  alternated.  The  results  obtained  in  this  way  are  true 
mixtures  of  coloured  light,  and  correspond  to  those  above 
given.  For  instance,  lines  of  cobalt-blue  and  chrome-yellow 
give  a white  or  yellowish-white,  but  no  trace  of  green  ; 
emerald-green  and  vermilion  furnish  when  treated  in  this 
way  a dull  yellow  ; ultramarine  and  vermilion,  a rich  red-pur- 
})le,  etc.  This  method  is  almost  the  only  practical  one  at  the 
disposal  of  the  artist  whereby  he  can  actually  mix,  not  pig- 
ments, but  masses  of  coloured  light.  In  this  connection  we 
are  reminded  of  an  interesting  0])inion  of  Ruskin  which  has 
some  bearing  on  our  subject.  The  author  of  “ IModern 
Painters,”  in  his  most  admirable  “ Elements  of  Drawing,” 
says  : “ Breaking  one  colour  in  small  points  through  or 
over  another  is  the  most  important  of  all  processes  in  good 
modern  oil  and  water-colour  j)ainting.  ...  In  distant  ef- 
fects of  a rich  subject,  wood  or  rippled  water  or  broken 
clouds,  much  may  be  done  by  touches  or  crumbling  dashes 
of  rather  dry  colour,  with  other  colours  afterward  put  cun- 
ningly into  the  interstices.  . . . And  note,  in  filling  up 
minute  interstices  of  this  kind,  that,  if  you  want  the  colour 
you  till  them  with  to  show  brightly,  it  is  better  to  put  a 
rather  positive  ]>oint  of  it,  with  a little  white  left  beside  or 
round  it,  in  the  interstice,  than  to  put  a pale  tint  of  the  col- 
our over  the  whole  interstice.  Yellow  or  orange  will  hardly 
show,  if  pale,  in  small  spaces  ; but  they  show  brightly  in 
tine  touches,  however  small,  with  white  beside  them.” 

This  last  method  of  mixing  coloured  light  is  one  which 
often  occurs  in  nature  ; the  tints  of  distant  objects  in  a 
landscape  are  often  blended  in  this  way,  and  produce  soft 
hues  which  were  not  originally  present.  Even  near  objects, 
if  numerous  and  of  small  dimension,  act  in  the  same  man- 
ner. Thus  the  colours  of  the  scant  herbage  on  a hillside 


ON  THE  MIXTURE  OF  COLOURS. 


141 


often  mingle  themselves  in  this  way  with  the  greenish-grey 
tints  of  the  mosses  and  the  brown  hues  of  the  dried  leaves  ; 
the  reddish-  or  purplish-brown  of  the  stems  of  small  bushes 
unites  at  a little  distance  with  their  shaded  green  foliage  ; 
and  in  numberless  other  instances,  such  as  the  upper  and 
lower  portions  of  mosses,  sunlit  and  shaded  grass-stalks,  and 
the  variegated  patches  of  colour  on  rocks  and  trunks  of 
trees,  the  same  principle  can  be  traced. 

There  is  another  mode  of  mingling  coloured  light,  which 
is  not  much  used  by  physicists,  though  it  is  of  constant  oc- 
currence in  nature.  We  refer  to  the  case  where  two  masses 
of  coloured  light  fall  simultaneously  on  the  same  object. 
Sunsets  furnish  the  grandest  examples  of  these  effects,  the 
objects  in  a landscape  being  at  the  same  time  illuminated 
by  the  blue  sky  and  the  orange  or  red  rays  of  the  sinking 
sun.  Minor  cases  happen  constantly  ; among  them  the 
commonest  is  where  a coloured  object  reflects  light  of  its 
own  tint  on  neighboring  objects,  thus  modifying  their  hues 
and  being  in  turn  modified  by  them.  The  white  or  grey 
walls  of  a room  are  often  very  wonderfully  tinted  by  col- 
oured light  which  is  cast  on  them  in  nebulous  patches  by 
the  carpet,  window-curtains,  or  other  coloured  objects  that 
happen  to  be  present.  In  all  cases  where  the  surface  re- 
ceiving the  manifold  illumination  is  white  or  grey,  or  but 
slightly  coloured,  the  laws  for  the  mixture  of  coloured 
light  which  have  been  explained  above  hold  good  ; when, 
however,  this  surface  has  a distinct  colour  of  its  own,  the 
phenomena  are  modified  in  a manner  which  will  presently 
be  noticed. 

We  pass  on  now  to  compare  the  results  which  are  ob- 
tained by  mixing  coloured  lights  with  those  which  are 
given  by  the  mixture  of  coloured  pigments.  It  was  for  a 
long  time  supposed  that  these  were  identical,  and  that  ex- 
periments on  mixtures  of  coloured  light  could  be  made 
with  the  aid  of  the  painter’s  palette.  Lambert  appears  to 
have  been  the  first  to  point  out  the  fact  that  the  results  in 


142 


MODERN  CHROMATICS. 


the  two  cases  are  not  always  identical.  Thus  the  celebrate 
experiment  of  combining  blue  with  yellow  light,  and  ob- 
tainino-  not  green  but  white,  was  first  made  by  him  wrcb  the 
apparatus  shown  in  Fig.  39,  page  110.  The  same  fact  was 
afterward  independently  discovered  by  Plateau,  and  finally 
bv  Helmholtz,  who,  using  it  as  a starting-point,  made  an  ex- 
amination of  the  whole  subject.  When  we  study  this  matter 
with  some  attention,  we  find  that  in  mixing  pigments  two 
dinercnt  effects  are  produced.  Suppose  we  mix  chrome- 
. yellow  and  ultramarine-blue,  both  iu  dry  powder.  It  we 
nib  this  mixture  on  paper,  we  shall  produce  a uniform  and 
rather  dull  green.  An  examination  even  with  a modeiately 
n'owcrfiil  microscope  will  fail  to  reveal  the  separate  parti- 
cles of  the  two  pigments.  Yet  we  know  that  theie  mus 
be  a superficial  layer,  made  up  of  a mosaic  work  of  blue 
. and  yellow  particles  placed  side  by  side.  These  two  sets  of 
particles  send  light  of  their  own  colour  to  the  eye,  y Inch 
there  undergoes  a true  mixture,  and  giycs  as  the  resultant 
hue  a yellowish-grey.  Thus  far  the  result  entirely  corre- 
sponds with  that  produced  by  mixing  two  masses  of  cob 
oiired  li"-ht.  The  second  and  more  important  effect  is 
broimht  "about  by  light  which  penetrates  two  or  more  lay- 
ers of  particles.  Here  the  light  undergoes  absorption  m 
the  manner  explained  iu  Chapter  VII.  : the  yellow  part.c  es 
absorb  the  blue  and  violet;  the  blue  particles,  the  led, 
orann-e,  and  vellow  rays.  The  green  light  is  absorbed  also 
by  both  sets  of  particles,  but  not  nearly  so  much  as  the 
oilier  rays.  From  all  this  it  follows  that  chrome-yellow 
and  uttramarine-bliie  jointly  absorb  all  the  colours  which 
are  present  iu  white  light,  except  green  ; hence  green  light, 
as  the  result,  is  reflected  back  from  the  surface,  and  reaches 
the  eye  of  the  obserxer.  This  green  light  is 
with  the  yellowish-grey  light  before  mentioned, 
pigments'  are  employed,  both  of  the  effects  just  desmbed 
ar'e  always  present.  It  tke  pigments  are  used  as  watert 
colours,  the  light  from  the  surface  is  diininished,  and  with 


ON  THE  MIXTURE  OE  COLOURS. 


143 


it  tlie  first  of  these  effects  ; and  a still  further  diminution 
of  it  takes  place  if  the  pigments  are  ground  up  in  oil. 
From  all  this  it  is  evident  that  by  mingling  tioo  pigments 
we  obtain  the  resultant  effect  of  two  acts  of  absorption  due 
to  the  two  pigments  : white  light  is  twice  subjected  to  the 
process  of  subtraction,  and  what  remains  over  is  the  col- 
oured light  which  finally  emerges  from  the  painted  surface. 
On  the  other  hand,  the  process  of  mixing  coloured  light  is 
essentially  one  of  addition  ; and,  this  being  so,  we  find  it 
quite  natural  that  the  results  given  by  these  two  methods 
should  never  be  identical,  and  often  should  differ  widely. 
From  this  it  follows  that  painters  can  not  in  many  cases 
directly  apply  knowledge  acquired  from  the  palette  to  the 
interpretation  of  chromatic  effects  produced  by  nature,  for 
these  latter  often  depend  to  a considerable  extent  on  the 


Fig.  58.— Two  Apertures  in  Black  Cardboard  covered  with  Eea  and  Green  Glass 
(natural  size.) 

mixing  of  masses  of  differently  coloured  light.  This  fact  is 
now  admitted  in  a general  way  by  intelligent  artists,  but 
probably  few  who  have  not  made  experiments  in  this  direc- 
tion fully  realize  how  wide  are  the  discrepancies  which  exist 
between  the  results  given  by  the  two  different  modes  of 
mixture.  A few  years  ago  Dove  described  a method  of 
studying  this  subject  with  the  aid  of  stained  glass  ; and,  as 
it  would  be  difficult  to  devise  a simpler  or  more  striking 
mode  of  making  such  experiments,  we  will  give  his  process 
in  full. 


7 


144 


MODERX  CHROMATICS. 


In  a blackened  piece  of  cardboard  two  apertures  are 
cut,  about  a third  of  an  inch  broad,  as  indicated  in  Fig.  58. 
Over  these  apertures  are  fastened  pieces  of  stained  glass- 
ier instance,  red  and  green  ; light  from  a white  cloud  is 
then  made  to  traverse  the  glasses.  At  P,  Fig.  59,  is  an 


IiG.  r)9. — Mode  of  usinsf  Dove’s  Apparatus.  R f»  is  canlboard  witli  red  and  green  glass; 
at  P is  the  prism  of  calc  spar. 

achromatic  prism  of  calc  s}>ar,  which  doubles  each  of  the 
little  patches  oi  coloured  light,  so  that  the  observer  on 
looking  through  the  prism  actually  secs  two  red  images  of 


exactly  equal  brightness,  and  also  two  similar  green  images. 
Xow,  by  revolving  tlie  calc-spar  prism  the  experimenter 
can  cause  one  of  the  red  images  to  OA^erlap  one  of  the 
green,  and  thus  it  is  ]mssible  to  mingle  the  red  and  green 


ON  THE  MIXTURE  OF  COLOURS. 


145 


light  from  the  coloured  glasses.  In  one  experiment  made 
by  the  present  writer  the  colour  of  this  mixture  was  orange 
(see  Fig.  60).  Upon  removing  the  glasses  from  the  instru- 
ment, placing  them  one  over  the  other  and  allowing  white 
light  to  pass  through  them,  the  effect  due  to  the  double 
absorption  manifested  itself  ; but  now  the  colour  of  the 
transmitted  light  was  not  orange,  or  even  brown,  but  dark 
green.  If  these  two  glasses  had  been  ground  to  powder, 
mixed  with  oil,  and  then  the  mixture  painted  on  canvas,  it 
would  have  exhibited  not  an  orange  but  a dark-green  hue. 
We  give  below  the  results  of  a set  of  experiments  which 
were  recently  made  by  the  writer,  and  which  illustrate  the 
differences  to  be  encountered  in  the  two  modes  of  proceed- 
ing : 

Results  given  by  Dove’s  Apparatus. 


Colours  of  Glass. 

Eesult  obtained  by  mixture 
of  Light. 

Result  obtained  by  Ab- 
sorption. 

Red  and  green 

Orange. 

Dark  green. 

*Red  and  green Y 

Pale  yellow. 

Black. 

Yellow  and  blue 

White. 

Fine  green. 

*Yellow  and  blue 

Pinkish-white. 

Fine  olive-green. 

Red  and  blue 

Violet-purple. 

Deep  red. 

Yellow  and  dark  purple.. . 

Yellow. 

Deep  orange. 

*Yellow  and  dark  purple. , 

. Pale  orange. 

Dark  brown. 

Purple  and  green 

White. 

Dark  green. 

Yellow  and  red ' 

Yellow,  slightly  orange. 

Deep  orange-red. 

* Yellow  and  red 

Orange. 

Red. 

Yellow  and  bluish-green..  . 

Yellow. 

Yellowish-green. 

* Yellow  and  blue-green...  . 

Yellowish-white. 

Rich  yellowish-green. 

**Yellow  and  blue-green . . 

Pale  greenish-yellotv. 

Olive-:  reel! . 

Purple  and  blue-green.  . . . 

Pale  blue-green. 

Dark  violet. 

Purple-violet  and  green . . . 

Pale  violet-blue. 

Black. 

These  are  not  experiments  selected  so  as  to  show  wide  dif- 
* Sample  more  deeply  tinted. 


146 


MODERN  CHROMATICS. 


ferences  ; they  comprise  the  entire  set  that  was  made  on 
the  occasion  referred  to,  and  are  simply  transcribed  from 
the  author’s  note-book.  Yet  it  will  be  seen  that  in  not  a 
single  case  do  the  two  methods  furnish  the  same  result ; 
and  as  a general  thing  they  differ  so  entirely  that  it  would 
be  quite  impossible  even  to  predict  the  nature  of  one  of  the 
sets  of  tints  from  a knowledge  of  the  other.  These  experi- 
ments, then,  exhibit  the  wide  difference  between  the  effects 
produced  by  mixture  of  light  and  the  absorption  of  light  ; 
but,  as  has  already  been  remarked,  when  pigments  are 
mixed  on  the  palette,  the  resultant  hue  depends  partly  on 
the  process  of  true  mixture  and  partly  on  that  of  absorp- 
tion, the  latter  of  course  predominating.  Hence  the  results 
in  the  table,  though  instructive,  are  not  necessarily  strictly 
applicable  to  the  painter's  palette,  which  is  best  studied  by 
another  method. 


Fig.  f.]  -Disk  for  sho^'iDg  the  Difference  between  mixing  Coloured  Light  and  Coloured 
Pigments.  The  outer  disk  is  painted  with  the  pure  pigments,  the  small  disk  with  a 
mixture  of  the  same  pigments. 


For  an  examination  of  this  matter  the  author  adopted 
the  following  mode  of  proceeding  : Two  tolerably  deep 
washes  of  water-colour  pigments  were  prepared— for  in- 
stance, vermilion  and  ultramarine-blue  with  which  two  of 
Maxwell’s  disks  were  separately  painted.  Afterward  an 
equal  number  of  drops  of  the  same  washes  v.^ere  mingled 
on  the  palette,  and  a third  and  smaller  disk  painted  with  this 
mixture.  The  disks  were  placed  on  a rotation  apparatus, 


ON  THE  MIXTURE  OF  COLOURS. 


147 


arranged  as  in  Fig.  61,  the  vermilion  and  ultramarine  cover- 
ing each  one  half  of  the  larger  disk  ; the  smaller  one,  ex- 
hibiting the  result  furnished  by  the  palette,  being  placed  in 
the  centre.  When  the  compound  disk  was  rotated,  the  col- 
ours of  its  outer  portion  underwent  true  mixture,  and  it  was 
easy  to  compare  the  resultant  tint  with  that  furnished  by 
the  palette.  In  the  experiment  referred  to  the  result  was 
as  follows  : The  larger  disk  became  tinted  red-purple, 
alongside  of  which  the  smaller  disk  seemed  grey,  so  dull 
and  inferior  was  its  colour.  The  real  colour  of  the  smaller 
disk  was  a dull  violet-purple.  It  will  be  noticed,  then,  that 
not  only  was  the  colour  much  darker  and  less  saturated,  but 
it  had  been  moved  from  a red-purple  to  a violet-purple. 
ISText,  in  order  to  ascertain  how  much  the  pigments  had 
been  darkened  by  mixture  on  the  palette  and  otherwise 
changed,  a black  disk  was  combined  with  the  vermilion  and 
ultramarine  disks,  and  various  amounts  of  black  introduced 
into  the  red-purple  mixture  by  rapid  rotation.  It  was  found 
impossible  in  this  way  to  bring  the  colour  of  the  larger  disk 
to  equality  with  that  of  the  smaller  one,  it  remaining  always 
too  saturated  in  hue.  Some  white  was  then  added  to  the 
large  disk,  and  equalization  finally  effected.  It  was  then 
found  that  twenty-one  parts  of  vermilion,  twenty  parts  of 
ultramarine,  with  fifty-one  parts  black  and  nine  parts  white, 
made  a tint  by  rotation  which  was  identical  with  that  given 
by  mixing  up  the  vermilion  and  ultramarine  on  the  palette. 
The  large  amount  of  black  which  it  was  necessary  to  add 
strikingly  illustrates  the  general  proposition  that  every 
mixture  of  pigments  on  the  painter’s  palette  is  a stride 
toward  blackness.  We  give  now  the  results  of  the  other 
experiments  : 


148 


MODERN  CHROMATICS. 


Table  showing 


the  Effects  of  mixing  Pigments  by  Rotation  and  on 
THE  Palette. 


Pigments. 

By  Rotation.  1 

On  the  Palette. 

Violet  (“violet-carmine”) 1 

Yellow-green  (Hooker’s  green) ) 

Yellowish-grey. 

Brown. 

Violet  (“violet-carmine”) 1 

Yellow  (gamboge) ^ 

Pale  yellowish- 
grey. 

Sepia-grey. 

Violet  (“violet-carmine”) j. 

Creen  (Prussian-blue  and  gamboge)..  . ) 

Greenish-grey. 

Grey. 

Violet  (“violet-carmine”) > 

Plue-grey. 

i Blue-grey. 

Violet  (“violet-carmine”) 1 

Pink-purple. 

Dull  icd-purplc. 

: Pale  greenish- 
I grey. 

Full  blue-grccn. 

Yellowish-or- 
ange (flesh-tint). 

I Brick-red. 

Pale  reddish 
(flesh-tint). 

Dark-red. 

It  will  lie  notice.!  that  in  only  one  case  do  the  results  of  the 
two  inethoils  ooinoi.lo  ; in  all  the  others  the  tints  from  the 
I, alette  are  not  only  much  darker,  hut  also  different.  Col- 
our-equations  were  then  obtained  for  the  eight  cases  abore 
iriveii  ill  exactly  the  manner  indicated  tor  vermilion  and 
riltramarine-hliie  ; an.l  as  they  present  the  facts  in  an  exa 
manner,  showing  how  much  black  it  was  necessary  to  intro- 
duce, and  how  far  the  proportions  of  the  two  eomponen 
colours  had  to  be  varied,  they  are  given  below ; 


Mixture  on  Palette. 

50  violet  -f  50  Hooker’s  green. . 

50  violet  + 60  gamboge 

50  violet  + 60  green 

50  violet  + 50  Prussian-blue. . 


Mixture  by  Kotation. 

21  violet  22-5  Hooker's  green  -b  4 
^ vermilion  4-  62’5  black. 

= 64  violet  + 20  gamboge  + 26  black. 

= 50  violet  + 18  green  + 32  black. 

= 47  violet  + 49  Prussian-blue  + 4 black. 


ON  THE  MIXTURE  OF  COLOURS. 


149 


50  violet  + 60  carmine 


50  gamboge  H-  50  Prussian-1 


-blue.  = 


j 12  yellow  (gamboge)  + 42  Prussian- 
( blue  -1-41  green  4-  4 black. 


50  vermilion  -f  60  ultramarine. . = 


i 21  vermilion  -|-  20  ultramarine  4-51 
( black  4-  9 white. 


50  Hooker’s  green  4-  50  carmine  == 
50  carmine  4-  50  green = 


j 23’6  yellow-green  (Hooker’s  green)  4-  8 
( ! carmine  4-  52  vermilion  4-16  black. 


= 50  carmine  4-  24  green  4-  26  black. 


It  will  be  noticed  that  the  amount  of  black  which  it  was 
necessary  to  introduce,  in  order  to  darken  the  true  mixture 
of  the  colours  so  as  to  match  the  mixture  of  the  pigmeyits, 
was  a very  variable  quantity,  ranging  from  four  to  fifty-two 
per  cent.  It  is  for  this  reason  that  artists  are  so  careful  in 
their  selection  of  pigments  for  the  production  of  definite 
tones,  particularly  when  they  are  to  be  luminous  in  quality. 
In  four  of  these  experiments  it  was  found  impossible  to 
bring  about  equality  without  adding  to  the  two  original 
constituents  a third  colour,  and  in  one  case  white  had  to  be 
added  ; so  that,  in  more  than  half  the  cases  examined,  the 
original  colours  were  found  incapable  cf  reproducing  by  a 
true  process  of  mixture  the  tint  obtained  on  the  palette 
without  the  aid  of  a foreign  element.  These  experiments 
serve,  then,  to  show  that  the  results  furnished  by  the  pa- 
lette can  not  be  relied  on  to  guide  us  in  the  interpretation 
or  study  of  effects  in  nature  depending  on  the  mixture  of 
coloured  light. 

We  propose  now  to  consider  the  results  which  are  pro- 
duced when  a coloured  surface  is  exposed  to  a coloured 
illumination  and  at  the  same  time  to  white  light.  Effects 
of  this  kind  are  very  common  in  nature,  and  are  frequently 
purposely  selected  by  artists  as  themes  ; in  a minor  degree 
they  are  always  present  to  some  extent,  even  when  we  seek 
to  avoid  them.  With  the  knowledge  which  we  have  now 
gained,  it  is  possible  for  us  to  recognize  the  fact  that  in 
such  cases  the  resultant  tint  of  the  surface  will  depend  on 


MODERN  CHROMATICS. 


150 

three  circumstances:  first,  on  the  colour  tvhich  it  assumes' 
owinc'  to  the  presence  of  the  white  light— that  is  to  say, 
which  it  has  owing  to  its  natural  or,  as  artists  call  it,  “_-Ocal 
colour”  ; secondly,  on  the  colour  commumcated  to  it  by 
that  portion  of  the  coloured  light  which  is  reflected  iina  - 
tcred  from  its  surface  ; and  to  these  there  must  he  added, 
thirdly,  the  elfects  produced  hy  the  coloured  light  which 
penetrates  below  the  surface,  and  is  reflected  after  iindei- 
ooinev  a certain  amount  of  absorption.  It  is  cpiite  easy  to 

uiake  satisfactory  e.x|.erinients  on  this  matter  with  the  aid 


of  a simple  aiTaugeinciit  contrived  by  the  aiithoi.  At  a 
distance  of  some  eight  or  ten  feet  from  a window,  a lens, 
with  a focal  length  of  about  five  inches,  is  placed  on  a 
table,  in  such  a wav  as  to  concentrate  the  white  light  from 
the  window.  In  front  of  the  lens  a plate  of  coloured  glass 
is  held,  and  the  result  is  that  we  obtain  a briglit  beam  o 
coloured  li^it.  which  can  be  thrown  on  any  coloured  sur- 
face such,  for  example,  as  painted  paper  (see  Fig.  6:.). 


ON  THE  MIXTURE  OF  COLOURS. 


151 


the  walls  of  the  room  are  white,  the  paper  will  at  the  sam4fe 
time  he  exposed  to  a white  illumination  ; and,  by  turning  it 
or  removing  it  farther  from  the  lens,  the  proportions  of  this 
double  illumination  can  be  varied  at  will.  We  will  describe 
two  experiments  that  were  made  with  this  arrangement  : 
Yellow  light  was  obtained  by  using  a plate  of  glass  which 
transmitted  light  having'  to  ,the  eye  a j^ure  yellow  hue, 
without  any  tendency  to  orange-yellow  or  greenish-yellow. 
In  this  beam  of  light  a piece  of  paper,  painted  with  a very 
intense,  deep  hue  of  artificial  ultramarine,  was  held.  The 
portion  illuminated  by  the  yellow  light  appeared  almost 
quite  white^  showing  that  a true  mixture  of  the  colours  had 
taken  place.  It  is  well  known  that  it  is  difficult  to  decide 
about  the  actual  colour  of  a spot  when  it  is  surrounded  by 
a coloured  field  ; hence,  in  order  to  avoid  deception  by  con- 
trast, it  is  well  in  these  experiments  to  observe  the  spot 
which  has  received  the  double  illumination  through  an 
aperture  cut  in  black  paper,  which  is  to  be  held  in  such  a 
way  as  to  permit  a view  only  of  this  spot.  This  precaution 
was  taken  in  the  present  case,  and  also  in  all  the  experi- 
ments that  are  given  below.  The  ultramarine  paper  was 
then  removed,  and  its  place  supplied  by  some  which  had 
been  painted  with  Prussian-blue.  The  spot  now  appeared 
of  a bright  green  colour,  which  proved  that  an  action  had 
taken  place  similar  to  that  produced  by  mixing  pigments 
on  the  palette.  The  explanation  is  as  follows  : The  yellow 
glass  transmits  yellow,  green,  orange,  and  red  light  ; and, 
as  was  explained  in  the  previous  chapter,  these  lights  taken 
together  make  a light  which  appears  to  us  yellow.  That 
jmrtion  of  this  compound  yellow  light  which  penetrates  the 
Prussian-blue  undergoes  a process  of  absorption  ; the  green 
constituent,  however,  is  not  absorbed,  and  consequently  is 
refiected  rather  abundantly  from  the  paper.  But  some  of 
the  yellow  light  is  reflected  unaltered  from  the  immediate 
surface  of  the  paper ; this  mixes  with  the  blue  light  (due  to 
the  white  illumination),  and  makes  white  ; so  that  what  we 


152 


MODERN  CHROMATICS. 


anally  have  is  green  mixed  with  more  or  less  white^  In 
the  experiment  where  the  ultramarine  paper  was  used,  no 
aoubt  Lme  absorption  took  place,  but  it  "’as  not  sulhcien 
to  modify  the  result  materially  ; the  blue  and  yellow  light 
simply  united,  ami  formed  white  light.  Below  are  gnen, 
in  the  form  of  tables,  a large  series  of  experiments  made 
recently  by  the  author;  and  an  exaniination  ot  them  u ill 
«how  that  for  the  most  ,,art  the  resultant  tint  depends 
rather  on  a true  mixture  of  coloured  lights,  and  that  absoip- 
,ion  acts  only  as  a minor  agent  in  modifying  the  results  . 


T.VIU.K  1. 


Yellow  L4jht  I'alllnn  on 
Paper  palntetl  with 

Cunuine  gave 

Verniilion  gave  

Omnge  * gave 

Chrome-yellow  gave.. . 

(himboge  gave 

Yellowish-green  \ gave 

(Ireent  

Bhie-green  ^ gave 

(\van-blue  H gave 

Trussian  blue  gave.  . . 

ritramarine-blue  gave 

Violet  • gave 

Turple-violet  **  ga'e. 

Rurplelt  gave 

Black  tt  gave 


Red-orange. 

Bright  orange  red. 

Bright  orange-yellow. 
Bright  yellow. 

Bright  yellow. 

Yellow. 

Bright  yellow-green. 
Yellow-green  ( whitish). 
Yellow-green. 

Bright  green. 

White. 

Rale  reddish  tint. 
Orange  (whitish). 
Orange. 

Yellow. 


♦ Mixture  of  red  lead  and  Indian-yellow. 

I Mixture  of  gamboge  and  Brussian-blue. 

+ Mixture  of  emerald-green  with  a little 
I Milre  ofcmerald-groon  .ith  a lithe  cobalt-blue. 
I Mixture  of  cobalt-bluc  and  emerald-green. 

Hoffmann's  violet  B.  B. 

« Hoffmann's  violet  B.  B.  and  carmine, 
tt  Hoffmann's  violet  B.  B.  and  carmine. 

Lampblack. 


ON  THE  MIXTURE  OF  COLOURS. 


153 


Table  II. 

Eed  Light  falling  on 
Paper  painted  with 

Carmine  gave Red. 

Vermilion  gave Bright  red. 

Orange  gave Red-orange  and  scarlet. 

Chrome-yellow  gave Orange. 

Gamboge  gave Orange. 

Yellowish-green  gave.  Yellow  and  orange. 

Green  gave Yellow  and  orange  (whitish). 

Blue-green  gave Nearly  white. 

Cyan-blue  gave Grey. 

Prussian-blue  gave Red-purple  or  blue-violet. 

Ultramarine-blue  gave Red-purple  or  blue- violet. 

Violet  gave Red-purple. 

Purple-violet  gave Red-purple. 

Purple  gave Purple-red  or  red. 

Black  gave Dark  red. 

Table  III. 

Green  Light  falling  on 
Paper  painted  with 

Carmine  gave Dull  yellow. 

Vermilion  gave Dull  yellow  or  greenish-yellow. 

Orange  gave -<....  Yellow  and  greenish-yellow. 

Chrome-yellow  gave.  . . , Yellowish-green. 

Gamboge  gave Yellowish-green. 

Yellowish-green  gave.  . . Yellowish-green. 

Green  gave Bright  green. 

Blue-green  gave Green. 

Cyan-blue  gave Blue-green. 

Prussian-blue  gave Blue-green,  cyan-blue. 

Ultramarine-blue  gave . . Cyan-blue,  blue. 

Violet  gave Cyan-blue,  blue,  violet-blue  (all  whitish). 

Purple-violet  gave Pale  blue-green,  pale  blue. 

Purple  gave Greenish-grey,  grey,  reddish-grey. 

Black  gave Dark  green. 

Table  IV. 

Blue  Light  falling  on 
Paper  painted  with 

Carmine  gives Purple. 

Vermilion  gives Red-purple. 

Orange  gives Wliitish-purplc. 


154 


MODERN  CHROMATICS. 


Chrome-yellow  gives  . . . 

Gamboge  gives 

Yellowish-green  gives.  . 

Green  gives 

Blue-green  gives 

Cyan-blue  gives 

Prussian-blue  gives 

Ultramarine-blue  gives  . 

Violet  gives 

Purple-violet  gives 

Purple  gives 

Black  gives 


Yellowish-grey,  greenish-grey. 
Yellowish-grey,  greenish-grey. 
Blue-grey. 

Blue-green,  cyan-blue. 
Cyan-blue,  blue. 

Blue. 

Blue. 

Blue. 

Ultramarine,  violet-blue. 
Blue-violet. 

Violet-blue,  purple-violet. 
Dark  blue. 


Tlic^o  experiments,  tnken  ns  n wliole,  show  that,  m calcn- 
Intin-  for  tlie  effects  itroanccff  l.y  illuminating  colourerl 
surfaces  l.v  colonreff  light,  we  must  be  guulea  mainly  by 
the  laws  which  govern  mixtures  of  coloured  lights,  rather 
th  in  by  those  which  can  be  deduced  from  experience  with 
pi..-ments  ; they  are  certainly  useful  in  teaching  us,  when 
studying  from  nature,  fearlessly  to  follow  oven  the  most 
evanescent  indicatiotis  of  the  eye,  utterly  regardless  of  the 
fact  that  they  disobey  laws  which  we  have  learned  from 
the  palette. 


We  pass  on  now  to  consider  the  changes  in  tint  which 
take  place  when  coloured  sttrfaces  arc  illuminated  by  lamp- 
li<dit  or  gasdight.  If  we  undertake  to  make  experiments  in 
this  direction ‘simply  by  viewing  coloured  surfaces  by  lainp- 
licrht  in  a room  illuminated  with  it,  correct  results  can  not 
he  obtained  ; for  by  this  very  method  we  have  practically 
rendered  otirselves  colottr-blind  to  a certain  extent,  and 
have  become  incapable  of  judging  correctly  of  quite  a series 
of  hues.  G.as-light  is  deficient  in  the  violet,  blue,  and  blu- 
ish-green rays;  hence  its  result.ant  tint  is  not  white,  but 
orange-yellow.  If  we  are  immersed  in  this  light,  it  will 
appear  to  us  white,  and  our  judgment  of  all  colours  will  be 
more  or  less  disturbed  : yellow  surfaces  will  appear  white 
or  whitish  ; blue  surfaces,  more  greyish-blue,  or,  if  pale. 


ON  THE  MIXTURE  OF  COLOURS. 


155 


even  pure  grey.  The  actual  changes  effected  by  artificial 
illumination  may  readily  be  studied  by  the  following  sim- 
ple method,  contrived  by  the  author  : A camera-obscura  is 
placed  in  a room  illuminated  by  ordinary  daylight  ; in 
front  of  it,  at  a short  distance,  is  placed  a gas-flame  or  lamp- 
flame,  in  such  a way  that  the  lens  of  the  camera  is  capable 
of  forming  an  image  of  it  of,  about  half  the  natural  size. 
(See  Fig.  63.)  This  image  is  now  allowed  to  fall  on  col- 
oured stuffs  or  on  coloured  paper  placed  behind  the  lens  of 
the  camera  at  S ; it  can  be  viewed,  as  indicated,  through 


Fig.  63.— Liglit  from  Gas-flame  is  concentrated  by  Lens  of  Camera  and  falls  on  Coloured 

Paper. 


the  top  of  the  camera,  and  the  resultant  tint  noted.  In 
the  experiments  made  by  the  author  a gas-flame  was  em- 
ployed, along  with  a set  of  painted  disks,  representing  the 
principal  colours.  The  disks,  fourteen  in  number,  were  the 
same  as  described  in  the  following  chapter,  and  constituted 
together  seven  pairs,  the  colours  of  which  were  comple- 
mentary, two  and  two.  Below  are  the  results  : 

1.  A carmine  disk  when  illuminated  by  the  gas-flame 
assumed  an  intense  red  hue,  even  more  brilliant  than  by 
daylight  ; the  complementary  disk,  painted  blue-green, 
appeared  of  a yellowish-green,  not  saturated,  but  rather  jiale. 


156 


MODERN  CHROMATICS. 


•2.  Vermilion  appeared  of  an  intense  fiery  red  ; its  com- 
plement, green-blue,  lost  in  strength,  and  became  yellowish- 
green  and  rather  pale. 

3.  Orange  appeared  brilliant  ; cyan-blue,  the  comple- 
ment, became  greenish-yellow  and  lost  in  saturation. 

4.  Yellow  became  brilliant,  showing  a tendency  toward 
orange  ; its  complement,  blue,  appeared  white,  or  rather 
pure  grey.  On  the  same  occasion  disks  painted  with 
chronie-ycdlow  were  examined  ; two  of  them  were  rendered 
somewhat  orange-yellow  ; the  third  was  brought  to  almost 
a full  orange  hue*  by  the  gas-light.  A disk  painted  with 
gamboge-yellow  ac<piired  something  of  an  orange  tint  under 
the  gas-light. 

5.  Oreenish-yellow  was  brought  to  a pure  yellow  ; its 
complement,  artificial  ultramarine-blue,  appeared  violet. 

6.  -yellow  became  pure  yellow  ; its  comple- 
ment, violet,  was  converted  into  a strong  red-puri)le. 

7.  Full  green  passed  into  a bright,  strong  yello^\ish- 
green  ; its  complement,  purple,  assumed  an  intense  pur- 
plish-red hue,  displaying  less  blue  than  by  daylight. 

dliese  are  the  actual  changes  produced  by  the  artificial 
illumination  as  they  ap|)eared  to  an  eye  placed  in  ordinary 
daylight,  and  consequently  able  correctly  to  note  the  sev- 
eral tints.  When  the  disks  were  examined  at  night  by  gas- 
light, in  many  cases  a different  result  was  reached.  The 
carmine  and  Vermilion  disks  still  appeared  very  brilliant, 
the  tint  in  the  case  of  the  former  being  a pure  red,  Avhile 
the  vermilion  showed  a tendency  to  red-orange.  The  orange 
disk  seemed  to  be  changed  in  tint  to  a redder  orange  hue  ; 
the  yellow,  on  the  contrary,  appeared  paler.  The  greenish- 
yellow  disks  did  not  show  much  change.  The  full  green 
was  intense,  appearing  perhaps  more  bluish  than  by  day- 
light. Blue-green  was  liable  to  be  confused  with  blue, 
cyanTdue  and  blue  with  green  ; artificial  ultramarine-blue 
appeared  more  purplish  than  by  daylight  ; violet  became 
purple,  and  purple  a very  red  purple.  Some  other  disks 


ON  THE  MIXTURE  OF  COLOURS. 


157 


were  also  examined  on  this  occasion  : gamboge  and  chrome- 
yellow  showed  a loss  in  saturation,  looking  whitish  ; indigo 
appeared  dull  greenish-grey  ; Prussian-blue  was  confused 
with  blue-green  ; genuine  ultramarine-blue  still  was  always 
blue  with  a slight  tendency  to  purple  ; cobalt-blue  exhib- 
ited this  same  tendency,  which  reached  a maximum  in 
French-blue.  All  the  blues  'appeared  much  duller  and 
greyer  than  by  daylight. 

By  comparing  these  two  sets  of  experiments,  it  will  be 
seen  how  greatly  the  judgment  of  colour  was  influenced  by 
the  circumstance  that  the  prevailing  illumination  was  yel- 
low, and  that  hence  a certain  shade  of  yellow  stood  for 
white,  and  gave  a false  standard  to  which  all  the  colours 
were  referred.  This  was  particularly  noticeable  in  the  case 
of  the  yellow  disks  ; in  point  of  fact,  as  the  first  set  of  ob- 
servations showed,  they  reflected  to  the  eye  much  yellow 
light,  and,  as  far  as  the  mere  physical  action  went,  ought 
to  have  produced  the  sensation  of  a strong,  brilliant  yellow 
hue  ; but,  as  all  surfaces  which  professed  to  be  white  were 
really  (owing  to  the  gas-light)  yellow,  this  competition 
caused  the  yellow  disks  to  appear  pale.  Another  case  illus- 
trates this  disturbed  judgment  even  better.  In  the  first  set 
of  experiments  it  was  found  that  the  blue  disk  when  illu- 
minated by  gas-light  really  assumed  a pure  grey  hue  with- 
out any  trace  of  blue  ; but  at  night,  although  it  must  have 
sent  to  the  eye  this  same  pure  grey  light,  it  always  appeared 
either  blue,  greenish-blue,  or  bluish-green  ; in  other  words, 
the  blue  disk,  when  held  near  the  gas-flame,  sent  to  the  eye 
white  light,  which  appeared  hlue^  by  contrast  with  the  pre- 
vailing yellow  illumination.  It  is  hardly  necessary  to  add 
that  these  causes  affect  our  judgment  of  paintings  and  dec- 
orations at  night  to  a very  considerable  degree,  the  blues 
being  rendered  less  conspicuous,  the  blue-greys  being  mostly 
abolished,  and  the  yellows  losing  in  apparent  intensity. 
Genuine  ultramarine-blue  is  less  affected  tlian  the  other 
blues,  cobalt  and  artificial  ultramarine-blue  becoming  pur- 


158 


MODERN  CHROMATICS. 


plisb,  and  Prussian-blue  quite  greenish.  It  hence  follows 
that  paintings  in  which  the  blue  tones  are  rather  overdone 
appear  often  better  by  gas-light ; but  this  is  hardly  the  case 
when  the  green  hues  are  of  somewhat  too  great  strength, 
the  evil  seeming  often  to  be  exaggerated  by  artificial  illumi- 
nation, which  must  of  course  be  due  to  an  act  of  the  judg- 
ment, as  the  greens  really  assume  a more  yellowish  appear- 
ance by  gas-light  or  lamp-light,  as  was  proved  by  the  first 
set  of  experiments.  From  this  it  follows  that,  if  the  chro- 
matic composition  of  a picture  is  quite  right  for  daylight, 
it  will  l)e  more  or  less  wrong  when  viewed  by  gas-light  ; 
hence  it  would  be  desirable  to  illuminate  picture  galleries 
at  night  witli  some  kind  of  artiticial  white  light,  a problem 
which  the  future  will  no  doubt  solve. 

All  the  appearances  which  have  thus  far  been  considered 
could  be  satisfactorily  observed  and  studied  by  a person 
possessed  of  only  a single  eye.  Let  us  now  turn  our  atten- 
tion for  a moment  to  some  very  remarkable  phenomena 
Avhich  occur  Avhen  different  colours  are  presented  to  the 
riglit  and  left  eye.  This  is  a case  which  happens  occasion- 
ally, particularly  when  we  look  at  the  reflection  from  pol- 
ished surfaces  or  from  water.  In  order  to  simplify  matteis, 
let  us  take  a case  where,  for  instance,  yellow  light  is  pre- 
sented to  the  right  and  blue  light  to  the  left  eye.  It  is 
very  easy  to  make  an  experiment  of  this  kind  with  the  aid 
of  the  stereoscope.  Selecting  one  of  the  common  paper 
slides  we  colour  it  as  indicated  in  Fig.  64,  and  then  view 
it  with  the  stereoscope.  AYe  have  already  seen  that  blue 
and  vellow  light  when  presented  to  the  same  ep  undergo 
mixture  on  the  retina  and  produce  the  sensation  we  call 
white.  This  would  lead  us  very  naturally  to  suppose  that, 
if  blue  lio-ht  were  presented  to  the  right  eye  and  yellow  to 
the  left,  the  two  sensations  would  be  united  in  the  brain 
and  would  call  up  that  of  white.  The  effect  is,  however, 
of  a much  more  complicated  character.  ^ lewed  in  the 


ON  THE  MIXTURE  OF  COLOURS. 


159 


stereoscope  the  figure  will  appear  at  one  moment  blue, 
then  yellow,  as  though  it  had  no  permanent  colour  of  its 
own  ; sometimes,  again,  the  observer  seems  to  see  one  col- 
our through  the  other,  and  is  distinctly  conscious  of  the 
presence  of  both  occupying  apparently  the  same  place,  thus 
giving  rise  to  the  idea  that  the  object  might  have  at  the 
same  time  two  distinct  colours'  Meanwhile  the  little  draw- 
ing assumes  a highly  lustrous  appearance,  as  though  it  were 
made  of  polished  glass  ; this  is  quite  beautiful,  and  strikes 
with  some  astonishment  those  who  see  it  for  the  first  time. 
After  some  little  practice  has  been  gained,  the  blue  and 
yellow  colours  will  melt  into  a lustrous  blue-grey  or  pure 
grey  tint  now  and  then  for  a few  seconds,  when  again  the 


F 

YEL. 

/ 

\ 

BLUE 

/ 

Y. 

BLUE 

Y. 

B. 

YELLOW 

B. 

/ 

YEL. 

\ 

/ 

BLUE 

\ 

Fig.  64.— Slide  for  the  Stereoscope,  the  Eight  and  Left  Hand  being  differently  coloured. 

contradictory  and  confusing  phantoms  just  mentioned  will 
make  their  appearance.  Taken  altogether,  the  eifect  is 
quite  wonderful,  and  suggestive  of  something  like  a new 
sensation.  There  has  been  a good  deal  of  controversy  as 
to  whether  a true  blending  or  mixture  of  the  two  colours 
actually  does  take  place  in  the  brain.  The  experiments  of 
De  Haldat  and  Dove,  and  afterward  of  Lubeck,  Foucault, 
and  Regnault,  all  point  to  this  result.  The  results  obtained 
by  the  author  are  also  favorable  to  this  view.  But  it  must 
be  confessed  that  tlie  mixture  obtained  by  this  method  dif- 
fers in  one  respect  from  those  previously  described.  For, 
when  coloured  light  is  mixed  with  tlie  aid  of  rotating  disks 
or  by  Lambert’s  jnethod,  we  see  only  the  resultant  tint,  the 


100 


MODERN  CHROMATICS. 


two  components  disappearing  entirely  to  give  place  to  it. 
On  the  other  hand,  in  this  binocular  mixture  of  colours,  the 
presence  of  each  of  the  original  colours  is  all  the  while  to 
some  extent  felt,  and  we  are  disposed  to  say  that  we  see  a 
neutral  or  grey  hue  which  has  evidently  been  made  out  of 
blue  and  ^yellow.  Careful  experiments  by  the  author 
proved  that  the  tint  of  the  true  mixture  often  differed  from 
that  obtained  by  the  use  of  the  stereoscope  ; colours  which 
vvx*re  jiale,  however,  united  more  readily  than  intense  ones, 
and  gave  less  <livergent  results.*  The  binocular  mixture  of 
colours  always  produces  more  or  less  lustre  ; it  is  not  even 
necessary  to  employ  distinct  colours,  the  same  effect  being 
brought  about  by  tlie  mixture  of  a light  and  dark  shade  of 
the  same  colour,  or  simply  by  the  binocular  union  of  white 
and  black,  as  was  shown  by  Dove.  The  lustrous  appear- 
ance of  waves,  ripples,  and  broken  retlections  in  water  is  in 
each  case  mainly  produced  in  this  way,  and  hence,  stiictly 
speaking,  can  not  be  imitated  by  artists,  who  are  necessarily 
obliged  to  present  the  same  colours,  the  same  light  or  dark 
shades,  impartially  to  both  eyes.  It  is  for  reasons  similar 
to  the  above  that  a somewhat  lustrous  appearance  is  com- 
municated to  an  oil  painting  by  varnish,  or  to  a water- 
colour drawing  by  glass  ; the  eye  sees  the  picture  through 
the  light  slightly  reflected  from  the  glass  or  varnish,  and  is 
enabled  ap]>arently  to  penetrate  beneath  the  mere  surface 
of  the  pigment,  and  this  slight  illusion  falls  in  with  and 
helps  the  design  of  the  artist. 


* “ Amevic:m  Journal  of  Science,”  May,  1865. 


CHAPTER  XI. 


COMPLEMENTAR Y COLO URS, 

In  the  previous  chapter  we  found  that  the  mixture  of 
two  masses  of  coloured  light  in  some  cases  produced  white 
light  ; this  was,  for  example,  true  of  mixtures  of  ultrama- 
rine-blue and  yellow,  or  of  red  and  greenish-blue.  Any 
two  colours  which  by  their  union  produce  white  light  are 
called  complementary.  An  accurate  knowledge  of  the 
nature  and  appearance  of  the  complementary  colours  is 
important  for  artistic  purposes,  since  these  colours  furnish 
the  strongest  possible  contrasts.  The  best,  in  fact  the 
only,  method  of  becoming  acquainted  with  the  appearance 
of  colours  which  are  complementary  is  by  actually  studying 
them  with  the  aid  of  suitable  apparatus.  The  results  thus 
obtained  should  be  at  the  time  registered,  not  in  writing,  but 
by  imitating  as  far  as  possible  the  actual  tints  with  brush 
and  palette.  By  the  aid  of  polarized  light  it  is  possible  to 
produce  with  ease  and  certainty  a large  series  of  colours 
which  are  truly  complementary.  There  are  quite  a number 
of  instruments  for  accomplishing  this,  but  perhaps  the  sim- 
plest and  best  is  that  which  was  contrived  by  Brlicke  for 
this  express  purpose,  and  called  by  him  a schistoscope.  (See 
Fig.  65.)  This  little  apparatus  is  merely  a combination  of 
a low-power  simple’  microscope  with  a polariscope,  and  can 
easily  be  constructed.  Starting  from  below,  P is  a piece  of 
white  cardboard,  which  is  fastened  to  the  stand  as  indicated, 
and  is  consequently  capable  of  being  turned  so  as  to  reflect 
upward  more  or  less  white  light,  as  may  be  required.  X is 


163 


MODERN  CHROMATICS. 


a Nicol’s  prism,  which  polarizes  the  light  thus  reflected  ; it 
is  attached  to  a blackened  stage,  S.  At  xi  is  a small  square 
aperture  two  millimetres  in  size.  C is  a crystal  ot  calc 
spar  ■ L is  a convex  lens  of  a focus  sucli  as  to  cause  the  two 
images  of  the  square  opening  furnished  by  the  calc  spar 
iust  to  touch  each  other.  G and  G are  polished  wedges  of 
..lass,  the  angles  being  18°  ; for  rough  expenineiits  they 
may  be  dispensed  witli.  In  order  to  use  this  apparatus, 


tlie  tube  containing  the  calc  spar  is  to  be  moved  till  distinct 
vision  is  obtained  of  the  square  opening  in  the  stage  by 
the  eye  placed  at  L,  or  rather  of  the  hro  square  openings 
whieli  will  be  seen  ; the  tube  is  then  to  be  revolved  till  one 
of  these  images  disappears  entirely,  and  is  to  be  left  in  this 
position.  Besides  the  instrument  it  is  necessary  to  provide 
a large  number  of  thin  slips  of  selenite  or  crystalized  sul- 
phate of  lime.  If  a clear  transparent  piece  of  this  sub- 
stance is  procured,  it  will  be  easy  with  a'penknife  to  split 
off  two  or  three  hundred  thin  slips,  and  then  with  the 
aid  of  the  instrunieiit  to  select  those  which  are  worth  pr^- 


COMPLEMENTARY  COLOURS. 


163 


serving.  To  observe  the  colours  it  is  only  necessary  to  lay 
one  of  the  slips  on  the  stage  between  the  calc-spar  prism 
and  the  Mcol’s  prism,  and  then  to  turn  the  selenite  till  two 
brightly  coloured  squares  are  seen,  as  is  indicated  in  Fig. 
66.  These  two  squares  will  always  have  colours  which  are 


Fig.  C6.— Complementary  Colours  as  exhibited  by  the  Schistoscope  of  Briicke. 

complementary.  The  object  of  preparing  a large  number 
of  the  slips  of  selenite  is  the  production  of  a large  series  of 
complementary  tints.  The  thinner  slips  furnish  colours 
that  are  more  saturated  ; those  which  are  thick  give  pale 
colours,  or  colours  mixed  with  much  white  light.  It  will 
be  found  in  this  way  that  the  following  pairs  of  colours  are 
complementary  : 

Table  of  Complementary  Colours. 


Red Green-blue.* 

Orange Cyan-blue. 


* Following  Helmholtz,  most  writers  give  bluish-green  as  the  comple- 
ment to  red.  These  observations  of  Helmholtz  were  made  on  the  spectrum, 
the  field  being  small  and  only  a single  eye  employed.  Extended  observa- 
tions with  coloured  disks,  the  hue  of  which  can  be  studied  in  a more  natu- 
ral way  and  wdth  both  eyes  simultaneously,  have  convinced  the  present 
writer  that  the  complement  of  vermilion  is  a very  green  blue,  and  even  the 
complement  of  carmine  is  a very  green  blue  rather  than  a blue-green. 


1G4 


MODERN  CHROMATICS. 


Yellow  Ultramarine-blue.* 

Greenish-yellow ^ iolet. 

Green 

In  Fig.  OT  these  complementary  colours  are  arranged  in 
a circle.  "^They  are  of  course  only  a few  of  the  pairs  that 
can  be  noticed.  The  tints  situated  between  red  and  orange 
will  have  complements  lying  between  greenish-blue  and 
cyan-blue  ; those  between  orange  and  yellow,  again,  will 
tind  complements  between  cyan-blue  and  ultramarine-blue, 
etc.  As  before  remarked,  it  is  a good  plan  to  copy  the 
residts  with  water-colours  ; this  fixes  the  tacts  in  the  mem- 
ory far  better  than  mere  momentary  inspection. 


To  study  Hie  complementary  colours  with  the  aid  of  the 
spectrum  is  a much  more  troublesome  process ; stdl,  it  is  ot 
tercst  for  us  to  know  that  the  results  are  the  same  as 
whet,  polarise, 1 light  Is  employed.  Another  ’ 

serves  cousideratiou.  It  might  be  supposed  that  the  lumi- 
uositv  or  apparent  brightness  of  colours  which  are  co.uple- 
mentirv  would  be  the  same,  but  this  is  far  from  being  true  ; 

vellow.'for  example,  is  much  more  luminous  than  its  com- 
plementary blue,  and  the  difference  between  ^ 

mid  violet  is  still  greater.  ITelmholtz,  using  the  pure  col 

* The  comnlcnient  of  cenuine  ultraroarinc-blue  is  vellow  that  y*'' 
ficia.  IdtZriL  being;  greenish.yeilow  The  artific.ai  p.gmenl,  or 
French-blue,  is  a violet-blue. 


COMPLEMENTARY  COLOURS. 


165 


ours  of  tLe  spectrum,  ascertained  that  the  order  of  the 
luminosities  of  the  complementary  colours  is  about  that 
given  in  the  following  table  : 

Yellow. 

Orange  and  green  about  the  same. 

Red  and  cyan-blue^  about  the  same. 

Ultramarine-blue. 

Violet. 

From  this  it  follows  that  a violet  which  appears  to  the 
eye  quite  dark  is  able  to  balance  a bright  greenish-yellow, 
and  form  with  it  white,  and  the  same  is  true  of  ultrama- 
rine-blue and  yellow  ; red  and  its  complement  green-blue 
have  about  the  same  luminosities  ; orange  is  somewhat 
brighter  to  the  eye  than  its  complement  cyan-blue.  There 
is  another  way  of  stating  these  facts  : we  can  say  that  in 
mixtures  violet  has  a greater  power  of  saturation  than  any 
of  the  colours  ; next  follows  ultramarine-blue,  then  red  and 
cyan-blue,  etc. 

The  method  of  studying  complementary  colours  with 
the  aid  of  polarized  light  and  plates  of  selenite  is  simple  and 
beautiful,  but  there  are  many  cases  which  it  does  not  reach  ; 
above  all,  it  fails  to  furnish  us  with  the  means  of  ascertain- 
ing the  complementary  tints  in  just  those  instances  which  are 
of  particular  interest,  viz.,  the  pigments.  This  happens  be- 
cause the  colours  furnished  by  the  plates  of  selenite  are  for 
the  most  part  quite  like  those  of  the  spectrum,  only  mixed 
more  or  less  with  white  light.  We  should  seek  in  vain 
among  them  for  good  representatives  of  olive-greens  or 
chocolate-browns  and  many  other  common  tints.  One  of 
the  problems  that  present  themselves  most  frequently  is  to 
ascertain  the  complementary  colour  of  some  particular  pig- 
ment or  mixture  of  pigments.  For  the  rough  solution  of 
such  questions  a method  given  by  Dove  can  be  employed  : 
A small  square  of  paper,  an  inch  or  less  in  size,  is  to  be 
painted  with  the  pigment  in  question  and  placed  on  a sheet 


166 


MODERN  CHROMATICS. 


of  black  paper,  and  viewed  through  an  achromatized  j^rism 
of  calc  spar.  This  is  shown  in  Fig.  68,  and  has  the  property 


Fig.  G8. — Achromatic  Prism  of  Calc  Spar. 


of  furnishing  when  lield  before  the  eye  two  equally  bright 
images  of  objects  viewed  through  it.  It  is  used  in  this  ex- 
periment instead  of  a plain  calc-spar  prism,  because  it  gives 
a greater  separation  of  the  tAVO  images,  and  thus  allows  the 
employment  of  larger  squares  of  coloured  paper.  As  the 
tinding  of  the  colour  Avhich  is  complementary  to  any  gh^en 
one  depends  entirely  on  experiment,  a second  piece  of  pa- 
per, also  an  inch  square,  is  now  to  be  painted  with  the  col- 
our Avhich  it  is  supposed  will  be  complementary  to  the 
lirst,  and  the  two  painted  papers  are  to  be  combined  together 
with  the  aid  of  the  calc-spar  prism.  Let  us  suppose  that  we 
wish  to  obtain  the  complement  of  a dull  reddish-brown. 
The  red-brown  square  is  placed  on  the  black  paper  ; beside 
it  Ave  lay  a j)iece  painted  with  a dull  bluish-green  grey,  and 
arrange  matters  so  that  an  image  of  the  red-broAvn  paper 
falls  on  one  furnished  by  the  blue-grey  paper.  If  the  tAvo 
colours  are  complementary,  their  joint  image  will  be  white, 
or  rather  pure  grey.  If,  instead  of  pure  grey,  it  shows  a 
tendency  to  reddish-grey  or  bluish-grey,  the  colour  of  the 
second  slip  of  paper  must  be  modified  accordingly.  This 
operation  is  facilitated  by  constantly  comparing  the  tint 
obtained  Avith  that  of  a slip  of  pure  grey  paper,  placed  on 
the  same  sheet  of  black  paper.  When  the  process  is  fin- 
ished, the  appearance  will  be  that  indicated  in  Fig.  69. 

The  practical  objections  to  this  mode  of  experimenting 
are,  that  the  calc  spar  reduces  the  luminosities  of  the  col- 
oured papers,  and  that,  OAving  to  the  imperfect  means  of 


COMPLEMENTARY  COLOURS. 


167 


comparison,  one  is  too  apt  to  accept  as  pure  grey  any 
approximation  to  this  tint.  For  all  accurate  work  it  is  far 
better  to  employ  Maxwell’s  disks  in  the  manner  now  to  be 
described.*  Let  us  suppose  that  we  wish  to  obtain  the 


Fig.  69. — Eed-brown  and  Blue-grey  are  combined  in  the  central  Image,  and  form  a Pure 
Grey.  Below  is  a Grey  placed  for  comparison. 


complement  to  a somewhat  dark  vermilion-red.  The  de- 
tails in  an  actual  experiment  were  as  follows : A disk  was 
painted  with  the  tint  in  question,  and  combined  with  two 
others  painted  with  emerald-green  and  ultramarine-blue,  as 
it  was  known  beforehand  that  the  desired  colour  v/ould  be 
a bluish-green  of  some  kind.  See  Fig.  70,  which  shows 
also  smaller  black  and  white  disks  placed  on  the  same  axis 
for  the  purpose  of  obtaining  a pure  grey  for  comparison. 
It  will  be  noticed  that  the  red  colour  has  been  made  to  oc- 
cupy just  one  half  of  the  disk,  or  50  parts  ; the  remaining 
50  parts  are  to  be  divided  up  between  the  blue  and  green, 
as  is  found  by  experiment  necessary.  The  result  showed 


* For  an  account  of  Maxwell’s  disks,  see  previous  chapter. 


168 


MODERN  CHROMATICS. 


that  50  parts  of  red  were  neutralized  by  31  parts  of  emer- 
ald-green and  19  parts  of  artificial  ultramarine-blue  ; the 
three  colours  gave  a grey  identical  with  that  furnished  by 
13  parts  white  and  87  black.  Putting  this  in  the  form  of 
an  equation,  we  have  : 50  red  -f  31  em. -green  -f  19  ult.-blue 
= 13  white  + 87  black. 


The  next  operation  is  to  mix  emerald-green  and  ultra- 
marine-blue  in  the  proportion  of  31  to  19,  which  will  evi- 
dently give  us  the  correct  complement  of  our  red.  As 
these  two  colours  in  the  last  experiment  occupied  exactly  one 
half  of  the  disk,  it  evidently  will  be  necessary  to  double 
them  if  they  are  to  be  spread  over  a whole  disk  ; accord- 
ingly, we  combine  them  together,  taking  62  parts  of  the 
green  and  38  of  the  blue.  (See  Fig.  71.)  This  compound 
disk  when  set  in  rapid  rotation  gives  us  accurately  the  com- 
plementary colour  of  our  red.  It  is  seldom  in  practice  that 
so  complete  a result  as  this  can  be  obtained  ; for  it  is  evi- 
dent that,  if  the  red  colour  had  been  more  luminous,  it 
would  have  been  impossible  to  balance  50  parts  of  it  with 
50  parts  of  the  blue  and  green,  however  arranged  ; the 
resultant  tint  would  always  have  been  a reddish-graj. 
Conversely,  if  the  red  had  been  less  luminous,  a similar 


Fio.  TO. — Emerald-green  and  Ul- 
tramarine-blue Disks  arranged 
so  as  to  neutralize  Red  and  pro- 
duce with  it  a Pure  Gr^.  Cen- 
tral Black  and  White  Disk  for 
the  production  of  a Pure  Grey. 


Fig.  71. — Disks  of  Emerald-green 
and  Ultramarine-blue  arranged 
so  as  to  give  a Colour  Comple- 
mentary to  Red. 


COMPLEMENTARY  COLOURS. 


169 


difficulty  would  have  occurred  : the  resultant  tint  would 
have  always  been  somewhat  bluish  green  instead  of  pure 
grey. 

We  give  now  an  actual  experiment  which  illustrates  the 
way  in  which  the  matter  usually  falls  out,  and  shows  at  the 
same  time  the  nature  of  the  result  to  be  expected.  It  was 
desired  to  obtain  the  complement  to  a dull  yellow,  some- 
what like  the  tint  of  brown  pasteboard.  A disk  was  painted 
with  this  colour  and  combined  with  one  of  artificial  ultra- 
marine-blue,  the  small  black  and  white  disks  of  course 
being  present.  When  this  arrangement  was  set  in  rotation, 
it  was  found  impossible  to  produce  a pure  grey,  however 
the  proportions  of  the  blue  and  yellow  disks  were  varied  ; 
at  the  best  the  tint  furnished  was  a purplish-grey.  This  of 
course  indicated  the  necessity  of  adding  some  green  to  the 
blue  : a disk  of  emerald-green  was  now  added,  when  it  was 
found  that  43  parts  yellow,  combined  with  43  parts  ultra- 
marine-blue and  14  parts  emerald-green,  gave  a grey  identi- 
cal with  that  furnished  by  24  parts  of  white  and  76  black. 
The  equation  then  reads  : 41  yellow  -(-  45  blue  -|-  14  green 
= 24  white  -|-  76  black.  From  this  it  follows  that,  by  mix- 
ing ultramarine-blue  and  emerald-green  in  the  proportion 
of  45  to  14,  a colour  complementary  to  our  yellow  could  be 
obtained.  We  then  divide  up  100  in  this  ratio,  and  assign 
76-3  parts  to  the  blue  and  23*7  to  the  green,  combine  the 
blue  and  green  disks  in  this  ratio,  and  by  rotation  obtain 
the  complementary  colour,  which  is  a fine  blue.  This  blue 
is,  however,  somewhat  darker  than  the  true  complement  of 
our  yellow,  for  in  the  first  experiment  the  yellow  did  not 
occupy  fully  one  half  of  the  disk,  or  50  parts,  but  only  41 
parts  ; if  we  had  made  it  fill  half  the  disk,  the  other  col- 
ours would  not  have  been  luminous  enough  to  balance  it, 
and  grey  would  not  have  been  produced.  It  is  easy  for  us 
to  calculate  how  much  too  dark  the  tint  is  which  we  have 
obtained  as  the  complement  of  the  yellow  : if  we  call  the 
luminosity  of  the  true  complement  100,  then  that  which  we 


170 


MODERN  CHROMATICS. 


actually  obtain  is  69‘5.*  Hence,  in  using  this  complement, 
we  must  always  allow  for  the  fact  that  it  is  less  luminous 
than  the  true  complement  in  the  degree  above  indicated. 
Furthermore,  no  better  result  could  be  obtained  with  the 
disks  of  blue  and  green  which  were  used  ; to  improve  the 
result,  it  would  have  been  necessary  to  alter  them  so  that 
they  would  become  able  to  reflect  more  blue  and  green  light 
to  the  eye.  On  the  other  hand,  if  they  had  originally  re- 
flected too  much  green  and  blue  light,  this  might  have  been 
diminished  with  the  aid  of  a black  disk,  and  the  true  com- 
plement accurately  obtained.  Hence  it  follows  that  we  can 
obtain  the  true  complement  to  a given  colour  with  accu- 
racy only  in  those  cases  where  we  have  at  our  disposal  rep- 
resentatives of  this  comj)lementary  colour  which  are  suffi- 
ciently intense — that  is,  at  the  same  time  luminous  and 
saturated.  The  practical  effect  of  tliis  is,  that  we  can  not 
directly  obtain  the  conij)lementary  tints  of  the  most  intense 
of  the  warmer  pigments,  such  as  carmine,  vermilion,  red 
lead,  chrome-yellow ; the  colder  j)igments,  like  emerald- 
green,  cobalt-blue.  Prussian-blue,  ultramarine,  etc.,  all  fall- 
ing considerably  below  them  in  intensity. 

For  many  pur])Oses  it  is  convenient  to  j)Ossess  a set  of 
disks  arranged  in  pairs  and  representing  the  main  comple- 
n\entary  colours.  It  would  of  course  requ^^'^  much  time 
and  patience  to  construct  a set  in  which  the  colours  were 
quite  correct  in  the  matter  of  hue  and  also  in  that  of  lumi- 
nosity ; and  in  such  a set,  with  the  pigments  at  our  disposal, 
the  red  and  orange  hues  would  be  quite  dull,  and  the  yel- 
lows little  more  than  browns  or  olive-greens,  for  the  reason 
above  given.  The  author  recently  constructed  a set  in 
which  the  hues  were  nearly  correct,  and  the  luminosities  as 
favourable  as  could  be  obtained  without  too  much  expendi- 
ture of  time  and  trouble.  Their  relative  intensities  as 
pairs  were  also  determined,  as  well  as  the  amounts  of  white 
light  which  they  furnished  when  combined  in  pairs. 


* See  appendix  to  this  chapter. 


COMPLEMENTARY  COLOURS. 


171 


Table  of  Complementaey  Disks. 


Colour. 

Intensity. 

Colour. 

Intensity. 

Amount  of  white 
light  furnished 
hy  the  combi- 
nation. 

Carmine 

100 

Blue-green. . . . 

68-6 

25 

Vermilion 

100 

Green-blue  . . . 

66-2 

25-3 

Orange  

100 

Greenish-blue . 

887 

2'7-2 

Vollow 

100 

Blue 

645 

25'6 

Greenish-yellow.  . 

897 

French-blue  . . 

100 

28-9 

(rremis/i-yellow.  . 

887 

Violet 

100 

32-2 

Green 

100 

Purple 

86*9 

267 

It  will  be  noticed  that  an  attempt  was  made  to  have  the 
set  so  arranged  as  to  furnish  in  each  case,  as  far  as  possible, 
about  the  same  amount  of  white  light,  so  that  in  this  re- 
spect the  disks  should  have  nearly  the  same  rank.  For  an 
account  of  the  pigments  that  were  used,  the  reader  is  re- 
ferred to  the  appendix  to  the  present  chapter.  With  the 
aid  of  this  set  of  complementary  disks,  hundreds  or  even 
thousands  of  pairs  of  complementary  colours  can  be  quickly 
produced.  This  is  effected  by  combining  any  pair  either 
with  white  or  with  black,  or  with  both.  For  instance,  all 
the  reds  darker  than  carmine  can  be  obtained  by  combining 
the  carmine  disk  with  different  proportions  of  a black  disk, 
the  corresponding  complementary  colours  being  furnished 
by  the  blue-green  disk  similarly  treated  ; in  the  same  way, 
the  reds  paler  than  carmine  and  their  complements  are  to 
be  obtained  by  the  addition  of  a white  disk  ; and  finally, 
all  the  complementary  red  and  blue-green  greys  are  yielded 
by  adding  a white  and  a black  disk  to  either  of  the  two 
coloured  disks.  Hence  it  will  be  seen  that,  though  a set  of 
disks  of  this  kind  costs  some  trouble  at  the  start,  yet  after- 
ward it  more  than  repays  the  labour,  by  quickly  supplying 
us  with  a vast  range  of  colours  which  are  either  truly  com- 


172 


MODERN  CHROMATICS. 


plementary  in  all  respects,  or  defective  only  in  the  matter 
of  luminosity  to  a calculable  extent. 

We  come  now  to  a matter  which  at  first  sight  will  seem 
strange.  We  have  seen  that  every  colour  has  its  comple- 
mentary colour,  but  more  than  this  is  true  : every  colour 
has  many  different  complementary  colours.  This  may  best 
be  illustrated  by  an  experiment.  Let  us  suppose  we  wish 
to  study  the  colours  which  are  complementary  to  that  of 
our  green-blue  disk  : We  combine  this  disk  with  one  of 
vermilion,  to  which  it  is  complementary,  so  that  we  have 
50  parts  of  green-blue  and  as  much  vermilion  as  is  found 
necessary.  Now,  as  considerably  less  than  50  parts  of  ver- 
milion will  represent  the  complement  of  our  green-blue,  we 
fill  up  the  blank  space  left  by  the  vermilion  with  black. 
After  being  adjusted  so  as  to  give  a grey,  the  disk  was 
found  to  be  arranged  as  indicated  in  Fig.  72.  It  was  found, 
namely,  that  50  parts  of  green-blue  were  just  balanced  and 
Tieutralized  by  27  parts  of  vermilion,  leaving  23  parts  of 
the  disk  to  be  occupied  by  black.  To  render  visible  this 


Fm.  T2.— Green-blue  and  the  com- 
plementary amount  of  Vermilion. 


Fig.  78.— This  Disk  by  rotation 
gives  one  of  the  Cornplements 
of  Green-blue. 


complement  of  the  green-blue,  Ave  combine  a black  and  ver- 
milion disk  in  the  proportion  23  to  27,  or,  what  is  the  same 
thing,  in  that  of  46  to  54,  and  rotate  it.  (See  Fig.  73.)  This 
furnishes  us  with  a somewhat  dark  vermilion-red  ; it  is  one  of 
the  complements  of  the  green-blue.  If  we  now  replace  the 


COMPLEMENTARY  COLOURS. 


173 


black  in  Fig.  72  by  white,  the  proportions  still  remaining 
as  46  to  54,  we  produce  a light-reddish  flesh  tint,  quite  dif- 
ferent in  appearance  from  the  dark-red  colour  before  ob- 
tained, but  still  accurately  complementary  to  our  green- 
blue,  since  it  contains  the  same  amount  of  red.  If  the  46 
parts  of  black  are  gradually  replaced  by  white,  a series  of 
tints  will  be  obtained  differing  in  luminosity,  but  all  red- 
dish, and  all  complementary  to  the  same  green-blue.  The 
set  of  complementary  disks  above  described  furnishes  great 
facilities  for  studying  the  different  appearances  assumed  by 
pairs  of  complementary  colours  under  these  circumstances. 

Another  point  now  deserves  attention.  Suppose  that 
we  select  by  daylight  tw'o  painted  surfaces  with  colours 
that  are  strictly  complementary — for  instance,  red  and 
green-blue.  Afterward,  if  we  view  these  two  surfaces  by 
lamp-light  or  gas-light,  it  will  not  at  all  follow  that  the 
colours  will  still  neutralize  each  other  and  remain  comple- 
mentary. It  is  easy  to  experiment  on  this  matter  with  the 
aid  of  our  set  of  complementary  disks.  By  daylight  it  was 
found  that  41  parts  of  carmine  neutralized  59  parts  of  green- 
blue  and  gave  a true  grey  : by  gas-light  these  colours  were 
no  longer  complementary,  but,  in  the  above-mentioned  pro- 
portions, furnished  a pretty  strong  red-purple.  Experiment- 
ing still  by  gas-light,  the  red  was  reduced  to  29  parts  and 
the  green-blue  increased  to  71,  when  the  tint  of  the  mixture 
became  less  red,  but  still  neutralization  could  not  be  effect- 
ed : the  two  colours  had  by  gas-light  ceased  to  be  comple- 
mentary, and  it  was  found  necessary  to  add  13*5  parts  of 
green  to  reestablish  this  relation  between  them.  The  same 
fact  was  observed  with  the  following  pairs  of  complemen- 
tary colours  : 

Vermilion  and  green-blue. 

Orange  “ cyan-blue. 

Yellow  “ blue. 


174 


modern  chromatics. 


With  the  pair  greenish-yellow  and  ultramarine-blue,  the 
effect  was  reversed  ; it  was  necessary  by  gas-light  to  re- 
duce the  greenish-yellow  somewhat  and  to  replace  a portion 
of  it  by  orange,  dhe  pairs  greenish-yellow  and  violet, 
<a-een  and  purple,  remained  complementary  alike  by  day- 
fiaht  and  gas-light.  The  following  table  shows  the  results 
when  the  disks  were  arranged  so  as  to  appear  complemen- 
tary by  daylight  and  by  gas-light  : 


Colours.  1 

By  daylight. 

By  gas-light. 

Grey. 

Red-purple. 

-10’7  carmine  and  oj  o 

Purplish-red. 

36  vermilion  ana  ui  grctu-uio«- 

ii 

Purplish-red. 

47  orangG  unu  oo  tjau-uiuu 

ii 

Greyish-purple. 

39*2  yellow  auu  ou  o 

62*7  greenish-yellow  and  47-3  French- 

u 

1 Greenish-grey. 

U 

' Grey.f 

63  greenish-yeuow  ana  

i Grey.f . 

-iG‘0  green  ana  oo  o jiuipic 

Below  follow  the  proportions  when  the  disks  appeared 
complementary  by  gas-light  ; 


Colours. 

1 

By  daylight. 

By  gas-light. 

1 

29  carmine,  67  green-blue,  and  U green 

27  vermilion,  67  grecii-blue,  and  16  green  . . . 

37  orange,  60  cyan-blue,  and  13  green 

37-5  yellow,  66  blue,  and  6 6 green 

45  greenish-yellow,  48  French-blue,  7 orange. 
52  greenish-yeWovi  y 48  ^iolet 

Strong  green. 

u It 

Strong  green-grey. 

Greenish-grey. 

Purplish. 

Grey. 

Grey.f 

1 

! 1( 

i “ 

1 ^ 

47  green,  63  purpit 

These  changes  depend  on  two  causes.  First,  the  composi- 

* Vrtificial  ultramarine-blue. 

t Really  a dark  rellow,  which  appiared  by  gas-light  grey. 


COMPLEMENTARY  COLOURS. 


175 


tion  of  gas-light  is  different  from  that  of  white  light ; the 
violet,  blue,  and  greenish-blue  rays  in  gas-light  are  com- 
paratively feeble,  and,  owing  to  this  circumstance,  the  disks 
must  of  course  present  a different  appearance  when  illumi- 
nated with  it,  the  violet,  blue,  and  green-blue  pigments  ap- 
pearing relatively  darker.  This  circumstance  would,  how- 
ever, merely  require  us  to  use  more  of  these  hues,  and  would 
not  necessitate  the  introduction  of  foreign  colours.  The 
second  cause  is  that  bj^  gas-light  we  are  able  to  effect  neu- 
tralization only  when  the  mixture  of  the  two  colours  has  a 
tint  similar  to  that  of  the  general  illumination  itself,  which 
in  this  case  is  not  white,  but  yellow,  inclining  toward  or- 
ange. It  follows  from  these  experiments  that  if  red  or 
orange  is  to  be  contrasted  with  its  complement  by  gas-light, 
it  will  be  necessary  to  make  the  contrasting  colour  more 
greenish  than  would  be  allowable  by  daylight ; the  same  is 
true  to  a less  extent  of  orange-yellow  and  of  yellow  itself. 

Leaving  these  practical  matters  for  a moment,  let  us 
turn  our  attention  to  a couple  of  theoretical  points  which 
are  not  without  interest.  In  a previous  chapter  we  have 
seen  that  colour  varies  with  the  length  of  the  waves  of 
light ; knowing  this,  we  are  very  naturally  led  to  inquire 
whether  there  is  any  fixed  relation  between  the  lengths  of 
waves  which  produce  upon  us  the  sensations  of  comple- 
mentary colours.  Upon  studying  the  matter  with  the  help 
of  a chart  of  the  normal  spectrum,  we  find  that  no  such 
relation  exists,  owing  to  the  circumstance  that  the  change 
in  colour  in  different  parts  of  the  spectrum  is  not  directly 
proportional  to  the  change  in  wave-length,  as  was  pointed 
out  in  a previous  chapter.  Helmholtz  found  that  the  rela- 
tion which  does  exist  is  not  a fixed  one  for  all  the  different 
pairs  of  complementary  colours,  but  that  it  varies  consider- 
ably. With  some  of  the  pairs  this  relation  is  as  1 to  1*2, 
in  others  as  1 is  to  T333  ; or,  using  the  musical  notation, 
we  would  say  that  the  relation  varies  from  that  existing 
between  a note  and  its  fourth  to  that  between  a note  and 


176 


MODERN  CHROMATICS. 


its  diminishetl  third.  This  is  one  of  the  many  facts  which 
are  fatal  to  any  chromatic  theory  that  has  a musical  basis 
for  its  foundation. 

The  other  matter  demanding  our  attention  is  the  mode 
in  which  the  phenomena  of  complementary  colours  are  ex- 
plained by  the  theory  of  Thomas  Young.  In  a previous 
chapter  we  saw  that  a mixture  of  red,  green,  and  violet 
light,  when  presented  to  the  eye,  produced  the  sensation  of 
white  ; and  in  the  present  chapter  we  have  found  that  this 
same  sensation  can  be  produced  by  the  mixture  merely  of 
two  properly  selected  colours.  Now,  according  to  Young’s 
theory,  the  sensation  of  white  is  produced  when  the  three 
sets  of  nerve-tibrils  with  which  the  retina  is  provided  are 
stimulated  to  about -the  same  degree  of  activity  ; hence  it 
must  follow  that  two  colours  can  stimulate  all  the  three  sets 
of  nerves  as  elfectually  as  the  three  fundamental  colours. 
It  is  this  fact  that  we  are  called  on  to  account  for,  and  the 
explanation  in  the  principal  cases  is  as  follows  : 

Red  and  green-blue  are  complementary  colours,  because 
red  light  stimulates  the  red  nerves,  and  green-blue  light 
both  the  green  and  violet  nerves  ; the  joint  action  of  the 
three  sets  gives  white  light.  Orange  and  cyan-blue  is  the 
next  pair  : orange  light  sets  in  action  the  red  nerves  power- 
fully, also  somewhat  the  green  nerves  ; cyan-blue  sets  in 
action  the  green  and  the  violet  nerves  ; all  three  sets  of 
nerves  acting,  the  result  is  the  sensation  of  white.  The 
case  is  much  the  same  witli  yellow  and  genuine  ultramarine- 
blue  : both  colours  stimulate  two  sets  of  nerves  ; that  is, 
the  yellow  acts  on  the  red  and  green  nerves,  the  blue  on 
the  green  and  violet  nerves.  With  green  and  purple  the 
first  colour  acts  of  course  on  its  own  set  of  nerves,  the  sec- 
ond on  the  red  and  violet  nerves.  All  this  is  strictly  in 
accordance  with  the  principles  of  Young’s  theory,  as  will 
be  found  by  reference  to  the  chapter  in  which  it  is  treated. 

This  explanation  enables  us  to  understand  a fact  which 
otherwise  might  appear  quite  strange,  viz.  : that  if  we  take 


COMPLEMENTARY  COLOURS. 


177 


away  from  white  light  any  colour,  the  light  which  remains 
will  have  the  complementary  hue.  Thus,  if  we  strike  out 
from  white  light  the  orange  rays,  the  remainder  will  appear 
of  a rather  pale  cyan-blue.  The  table  of  complementary 
colours  explains  this  result  ; thus, 


Red  and  green-blue  make White. 

Orange  and  cyan-blue  make * White. 

Yellow  and  blue  make White. 

Green-yellow  and  violet  make White. 

Green  and  purple  make White. 


All  these  five  pairs  of  colours  are  present  in  white  light. 
If  we  remove  from  it  orange,  then  cyan-blue  is  the  only 
colour  which  is  not  neutralized  ; all  the  other  colours  bal- 
ance up  and  make  white  light,  which  mixes  with  and  pales 
the  uncomhined  cyan-blue.  The  explanation  is  the  same  in 
all  the  other  cases.  It  follows  from  this  that  the  comple- 
mentary colours  produced  by  the  method  of  striking  out  a 
colour  are  rendered  rather  pale  by  the  presence  of  a consid- 
erable amount  of  white  light.  This  is  the  reason  why  the 
complementary  colours  obtained  by  the  use  of  polarized 
light  are  always  rather  pale.  The  presence  of  this  white 
light,  as  will  be  shown  in  the  following  chapter,  actually 
somewhat  alters  the  tint  of  the  coloured  light  mixed  with 
it ; red  is  made  to  incline  to  purple,  orange  to  red,  pimple 
and  ultramarine  to  violet. 

Before  closing  this  chapter  it  may  be  well  to  make  some 
remarks  concerning  the  complement  of  pure  yellow,  and 
the  complements  of  the  several  varieties  of  the  more  com- 
mon blue  pigments.  In  different  works  the  complement  of 
yellow  is  given  as  indigo-blue,  ultramarine-blue,  or  simply 
as  blue.  Genuine  ultramarine-blue  is  complementary  to 
pure  yellow,  the  complement  of  artificial  ultramarine-blue 
being  a decidedly  greenish-yellow.  Gamboge  gives  a yel- 
low which  is  slightly  orange  ; its  complement  is  the  pig- 


178 


MODERN  CHROMATICS. 


merit  known  as  cobalt-blue.  The  complement  of  Prussian- 
blue  was  determined  and  found  by  the  author  to  be  a 
somewhat  orange-yellow  ; it  was  made  by  mixing  with  a 
rotating  disk  65  parts  of  pale  chrome-yellow  with  35  parts 
of  vermilion.  The  complement  of  indigo,  used  as  a water- 
colour pigment,  was  also  determined  with  care,  and  found 
to  correspond  cpdte  closely  with  that  of  Prussian-blue  ; it 
hence  follows  that  indigo  may  be  considered  to  be  the  same 
as  darkened  Prussian-blue,  and  not  to"  represent,  as  some 
authors  have  suggested,  darkened  ultramarine-blue.  The 
term  indigo  was  applied  by  Sir  Isaac  Newton  to  designate 
the  more  refrangible  blue  of  the  spectrum  ; to  this  it  does 
not  really  at  all  correspond  in  any  respect,  and  in  the  pres- 
ent work  the  term  ultramarine-blue  is  substituted  for  it.  If 
the  different  blues  be  arranged  in  the  order  of  the  spectrum, 
we  sliall  have  cyan-blue,  indigo  or  Prussian-blue,  cobalt- 
blue,  genuine  ultramarine  and  artificial  ultramarine,  the  last 
being  a violet-blue.  Among  the  yellow  pigments  the  some- 
what orange-tinted  chrome-yellow  is  complementary  to  in- 
digo and  Ih-ussian-blue  ; chrome-yellow  with  a still  more 
orange  hue  has  a complement  nearer  to  cyan-blue. 


APPENDIX  TO  CHAPTER  XI. 

The  mode  of  calculating  the  relative  intensities  of  pigments 
which  are  complementary  is  quite  simple,  and  is  here  illustrated  by 
an  example.  Let  us  suppose  that  25  parts  of  a certain  red  neutral- 
ize 75  parts  of  a green-blue.  The  compound  disk  will  then  appear 
as  in  Fig.  74.  It  is  evident  that  the  intensity  of  the  green-blue  is 
only  one  third  of  that  of  the  red,  since  it  takes  three  times  as  much 
green-blue  as  red  to  effect  neutralization.  Let  I be  the  greater  in- 
tensity and  P the  lesser ; then  we  have — 

25  I rr  Y5  r 


Making  the  greater  intensity  100,  we  have — 


APPENDIX  TO  CHAPTER  XI. 


179 


25  X 100  75  r 

r = 33-3 


Fig,  T4.— Disk  with  25  Parts  Red  and  75  Parts  Green-blue. 

That  is,  if  we  call  the  intensity  of  our  red  100,  that  of  the  green- 
blue  will  be  only  33*3.  In  the  case  given  in  the  present  chapter  we 
have — 

41  1 = 59  r 
41  X 100  = 59  r 
I'  = 69-5 


PIGMENTS  USED  IN  THE  SET  OF  COMPLEMENTAEY  DISKS. 


Carmine  as  a water-colour ; for  its  complementary  green-blue,  a 
mixture  of  cobalt-blue  and  emerald-green. 

Vermilion  as  a water-colour ; for  its  complement  the  same  as 
above,  the  proportions  being  changed. 

For  the  first  two  pairs,  then,  we  can  employ  two  of  our  most 
intense  and  saturated  pigments ; this,  however,  is  not  possible  with 
orange  and  yellow,  without  producing  disks  of  a rank  different  from 
the  preceding,  or  obtaining  disks  which  show  greater  differences  in 
luminosity  than  any  which  have  been  tolerated  in  the  table  given  in 
the  present  chapter.  Thus  a fine  orange  colour  was  mixed  from 
red  lead  and  Indian-yellow,  which  would  have  been  considered  by 
most  painters,  as  I suppose,  a fair  companion  for  the  carmine  and 
vermilion ; or,  if  objection  had  been  made,  it  would  have  been 
rather  to  its  want  of  intensity.  Placing  the  intensity  of  this  orange 
as  100,  the  intensity  of  its  complement  (made  of  cobalt-blue  and 
emerald-green)  was  only  47,  a figure  smaller  than  any  in  the  tabic. 


180 


MODERN  CHROMATICS. 


These  two  colours,  liowever,  furnished  a white  such  as  could  be 
obtained  by  mixing,  with  the  aid  of- disks,  36  parts  of  white  with  64 
of  black;  this  number  is  considerably  higher  than  those  allowed  in 
the  table.  The  combination  then  was  rejected,  because  it  was  faulty 
in  two  respects,  and  a dull-looking  orange  substituted  for  it.  This 
dull,  rather  poor-looking  orange  balanced  its  complementary  cyan- 
blue  well,  and  with  it  gave  27  per  cent,  of  white  light,  which  was 
fully  up  to  the  average,  and  proved  that  in  the  matter  of  luminosity 
it  belonged  in  the  set  rather  than  the  disk  just  mentioned. 

A similar  experience  was  encountered  with  yellow.  Two  beau- 
tiful disks  were  prepared  with  gamboge  and  cobalt-blue.  Setting 
the  intensity  of  the  gamboge  as  100,  that  of  the  cobalt  was  90, 
which  was  nearly  what  was  wanted.  The  combination,  however, 
gave  on  rotation  a white  which  was  about  100  per  cent,  too  bright, 
showing  that  the  two  disks  belonged  in  a set  such  as  would  be  fur- 
nished by  pigments  twice  as  bright  as  those  employed  by  me;  but 
no  such  pigments  exist. . This  is  only  another  illustration  of  the 
fact,  already  several  times  mentioned,  that  our  bright-yellow  pig- 
ments, such  as  gamboge,  chrome-yellow,  cadmium-yellow,  etc.,  can 
not  properly  be  reckoned  as  the  equal  companions  of  the  other  pig- 
ments ordinarily  found  on  the  painter’s  palette.  This  circumstance 
atfects  our  judgment,  and  we  are  surprised  at  the  lack  of  brilliancy 
of  the  yellow  space  even  in  the  prismatic  spectrum,  and  at  the  fact 
that  mixtures  of  red  and  green  light  produce  yellow  light  of  so  infe- 
rior a character.  On  the  other  hand,  the  possession  of  such  excep- 
tional pigments  as  the  bright  yellows  and  orange-yellows  enables 
the  artist  at  will  to  extend  his  scale  of  brilliancy  in  an  upward 
direction  much  farther  than  otherwise  would  be  possible. 

The  greenish-yellows  were  made  with  gamboge  mixed  with  a 
little  Prussian-blue,  the  pigments  being  laid,  not  on  drawing-paper, 
but  on  rather  absorbent  cardboard,  which  dulled  the  colours  to  a 
desirable  extent.  For  violet,  “Hoffmann’s  violet  B.  B.”  was  em- 
ployed, none  of  the  violet  pigments  used  by  artists  being  of  the 
slightest  use  on  account  of  their  very  dull  appearance  and  poverty 
in  the  matter  of  violet  light.  The  green  was  made  by  mixing  a 
little  chrome-yellow  with  emerald-green  ; the  purple  was  “ Hoff- 
mann’s violet  R.  R.  P.“ 


CHAPTER  XIL 


ON  THE  EFFECT  PRODUCED  ON  COLOUR  BY  A CHANGE 
IN  LUMINOSITY,  AND  BY  MIXING  IT  WITH  WHITE 
LIGHT 

In  our  study  thus  far  of  coloured  surfaces  it  has  been 
tacitly  assumed  that  their  action  on  the  eye  is  a constant 
one,  and  that  a red  surface,  for  example,  will  always  appear 
red  to  a healthy  eye  as  long  as  it  remains  visible.  In  point 
of  fact,  however,  this  is  not  quite  true,  for  it  is  found  that 
coloured  surfaces  undergo  changes  of  tint  when  they  are 
seen  under  a very  bright  or  very  feeble  illumination.  Ar- 
tists are  well  aware  that  scarlet  cloth  under  bright  sunshine 
approaches  orange  in  its  tint ; that  green  becomes  more 
yellowish  ; and  that,  in  general,  a bright  illumination  causes 
all  colours  to  tend  somewhat  toward  yellow  in  their  hues. 
Helmholtz,  Bezold,  Rutherfurd,  and  others  have  made  simi- 
lar observations  on  the  pure  colours  of  the  prismatic  spec- 
trum, and  have  found  that  even  they  undergo  changes 
analogous  to  those  just  indicated.  The  violet  of  the  spec- 
trum is  very  easily  affected  : when  it  is  feeble  (that  is, 
dark),  it  approaches  purple  in  its  hue  ; as  it  is  made  strong- 
er, the  colour  changes  to  blue,  and  finally  to  a whitish-grey 
with  a faint  tint  of  violet-blue.  The  changes  with  the 
ultramarine-blue  of  the  spectrum  follow  the  same  order, 
passing  first  into  sky-blue,  then  into  Avhitish-blue,  and  final- 
ly into  white.  Green  as  it  is  made  brighter  passes  into  yel- 
lowish-green, and  then  into  whitish-yellow  ; for  actual  con- 
version into  white  it  is  necessary  that  the  illumination  should 


182 


MODERN  CHROMATICS. 


be  dazzling.  Red  resists  these  changes  more  than  the  other 
colours  ; but,  if  it  be  made  quite  bright,  it  passes  into 
orange  and  then  into  bright  yellow. 

It  is  remarkable  that  these  changes  take  place  with  the 
pure  colours  of  the  spectrum  ; but  the  explanation,  accord- 
ing to  the  theory  of  Young  and  Helmholtz,  is  not  difficult. 
Let  us  illustrate  it  by  an  example,  taking  the  case  of  green 
light,  which,  as  we  have  seen,  acts  most  powerfully  on 
what  we  termed  the  green  nerves,  less  powerfully  on  the 
red  and  violet  nerves.  Now,  as  long  as  the  intensity  of  our 
green  light  is  small,  it  acts  almost  entirely  on  its  own  pecu- 
liar set  of  nerves  ; but,  when  the  green  light  is  made  bright- 
er, it  begins  to  set  into  action  also  the  red  and  to  a lesser 
extent  the  violet  nerves  ; the  result  of  this  is  that  the  sen- 
sation of  white  begins  to  be  mingled  with  that  of  green,  all 
three  sets  of  nerves  being  now  to  some  extent  in  action. 
As  in  this  process  the  violet  nerves  lag  behind,  the  main 
modilication  of  the  colour  at  this  stage  is  due  to  the  action 
of  the  red  nerves,  which  cause  it  to  appear  more  yellowish  ; 
hence  it  changes  first  to  a yellowish-green,  then  to  greenish- 
yelloAV,  and  finally,  if  the  light  is  very  bright,  to  a whitish- 
yellow.  Corresponding  to  this,  when  red  light  is  made 
very  bright,  the  red  and  the  green  nerves  are  set  into  ac- 
tion, the  result  being  that  the  colour  changes  in  appearance 
from  red  to  yellow.  In  this  case  the  violet  nerves  play  a 
secondary  part,  and  their  action  merely  causes  this  yellow 
to  appear  somewhat  whitish.  When  pure  violet  light  is 
made  quite  bright,  immediately  the  green  nerves  begin  to 
add  their  action  to  that  of  the  violet,  and  the  tint  quickly 
changes  from  violet  to  ultramarine-blue  ; the  red  nerves  are 
soon  also  stimulated,  and,  in  connection  with  the  green,  fur- 
nish the  sensation  of  yellow  ; this  yellow,  mixing  with  that 
of  the  ultramarine-blue  before  mentioned,  gives  as  a result- 
ant tint  a whitish-grey  Avith  a faint  tint  of  blue  or  violet- 
blue.  The  explanation  of  the  changes  which  the  interme- 
diate colours  of  the  spectrum  undergo  is  analogous  to  that 


EFFECT  ON  COLOUR  BY  CHANGE  IN  LUMINOSITY.  183 


just  given.  The  tendency  in  all  cases  is  to  the  production 
of  a yellowish- white,  or  to  a white,  if  the  coloured  light  be 
very  bright.  If  its  brightness  be  more  moderate,  the  col- 
our will  still  appear  paler  and  as  though  mixed  with  a cer- 
tain amount  of  yellow.  Artists,  by  taking  advantage  of 
these  facts,  are  able  to  represent  in  their  paintings  scenes 
under  high  degrees  of  illumination.  According  to  Aubert, 
the  whitest  white  paper  is  only  57  times  brighter  than  the 
darkest  black  paper  ; and  it  is  within  these  narrow  limits 
that  the  painter  is  compelled  to  execute  his  design  : hence 
the  necessity  of  employing  illusions  like  the  one  just  men- 
tioned. Many  effects  in  nature  are  beautiful  and  striking, 
as  much  on  account  of  their  high  degree  of  luminosity  as 
for  any  other  reason.  The  artist  is  not  able  to  transfer  to 
his  canvas  the  brightness,  which  in  this  case  is  really  the 
attractive  element ; but  by  the  use  of  pale  colours,  well 
modulated,  he  suggests  a flood  of  light,  and  we  are  delight- 
ed, not  so  much  with  the  pale  tints  as  with  the  recollections 
they  call  up. 

We  have  just  examined  the  remarkable  alterations  which 
the  pure  colours  of  the  spectrum  undergo  when  their  lumi- 
nosity is  made  very  great,  and  pass  now  to  the  changes 
which  occur  when  the  intensity  of  coloured  light  is  made 
very  feeble.  Von  Bezold  has  made  some  interesting  obser- 
vations of  this  character  on  the  colours  of  the  spectrum. 
With  a very  bright  prismatic  spectrum  he  was  able  to  see 
a pure  yellow  near  T>  and  a whitish-blue  near  F,  the  other 
colours  being  in  their  usual  positions.  When  the  illumina- 
tion was  only  moderately  bright,  the  yellow  space  dimin- 
ished and  became  very  narrow  ; the  ultramarine-blue  van- 
ished, and  was  replaced  by  violet.  With  less  illumination, 
the  orange-yellow  space  assumed  the  colour  of  red  lead,  and 
the  yellow  vanished,  being  replaced  by  a greenish  tint ; the 
cyan-blue  was  replaced  by  green,  the  blue  and  ultramarine- 
blue  by  violet.  The  spectrum  at  this  stage  presented  scarce- 
ly more  than  the  three  colours,  red,  green,  and  violet.  With 


184 


MODERN  CHROMATICS. 


a still  lower  illumination,  the  violet  vanished,  the  red  be- 
came red-brown,  and  the  green  was  visible  as  a pale-green 
tint ; then  the  red-brown  disappeared,  the  green  still  re- 
maining, though  very  feeble.  With  still  less  light,  even 
this  suggestion  of  colour  vanished,  and  the  light  appeared 
simply  grey. 

The  tendency  in  these  experiments  is  evidently  just  the 
reverse  of  what  was  observed  where  the  illumination  was 
very  bright.  In  that  case  the  coloured  light  as  it  increased 
in  brightness  gradually  set  all  three  sets  of  nerves  into  ac- 
tion, and  the  result  was  white  or  yellowish-white  ; but  here 
the  action  of  the  coloured  light  as  it  grows  feebler  is  more 
and  more  confined  to  a single  set  of  nerves.  From  this  it 
results  that  those  colour-sensations  which  are  due  to  the 
joint  action  of  two  sets  of  nerves  speedily  diminish  when 
the  colour  is  darkened,  and  are  replaced  by  the  primary 
sensations,  red,  green,  or  violet.  The  sensation  of  orange 
is  produced  by  those  light-waves  in  the  spectrum  which 
have  a length  such  as  to  enable  them  to  stimulate  the  red 
nerves  strongly  and  the  green  nerves  to  a lesser  degree  ; 
hence,  when  orange-coloured  light  is  made  very  weak,  it 
fails  to  act  on  the  green  nerves  while  still  feebly  stimulat- 
ing the  red,  and  consequently  the  sensation  of  orange  passes 
over  into  red.  For  similar  reasons  the  sensations  of  yellow 
and  greenish-yellow  pass  into  green,  as  do  also  those  of 
greenish-blue  and  cyan-blue  ; in  the  same  way  the  sensa- 
tions of  blue,  ultramarine-blue,  and  violet-blue  pass  into 
violet.  It  is  quite  evident  that  these  changes  furnish  an- 
other argument  in  favour  of  Young’s  theory  of  colour,  and 
also  tend  to  approve  the  selection  of  red,  green,  and  violet 
as  the  fundamental  colour-sensations. 

In  the  experiment  of  Von  Bezold  just  mentioned,  after 
the  spectrum  had  been  darkened  to  a certain  degree  only 
three  colours  remained — red,  green,  and  violet  ; this  dark 
red,  however,  as  far  as  sensation  goes,  is  somewhat  changed 
in  character,  and,  according  to  the  unpublished  experiments 


EFFECT  ON  COLOUR  BY  CHANGE  IN  LUMINOSITY.  185 


of  Charles  Pierce,  has  become  somewhat  purplish  ; the  same 
is  true  of  the  green,  which  is  more  bluish  ; the  violet  alone 
is  unchanged.  Kow,  just  these  same  effects  can  be  produced 
by  mixing  small  quantities  of  violet  with  red  or  green ; 
hence  the' final  effect  of  darkening  on  all  the  colours  of  the 
spectrum  is  virtually  to  mix  them  with  increasing  quantities 
of  violet  light.  The  cause  of  these  peculiar  changes,  ac- 
cording to  the  theory  of  Young  and  Helmholtz,  resides  in 
the  fact  that  the  violet  nerves  act  more  powerfully,  rela- 
tively to  the  red  and  green  nerves,  when  the  light  is  feeble. 
For  example,  if  we  present  to  the  eye  pure  green  light,  it 
will  stimulate  the  green  nerves  strongly,  the  red  and  violet 
to  a much  less  degree  : we  thus  obtain  a certain  sensation, 
and  call  it  green.  If  now  we  greatly  diminish  the  intensity 
of  the  green  light,  it  will  of  course  affect  the  green  nerves 
to  a minor  degree  ; but,  besides  this,  it  has  now  less  action 
on  the  red  than  on  the  violet  nerves,  so  that  virtually  we 
have  a mixture  of  green  and  violet,  which  will  cause  the 
green  to  appear  bluish-green.  The  same  explanation  holds 
good  for  the  red,  dull  red  light  producing  less  effect  on  the 
green  than  on  the  violet  nerves. 

The  change  which  colour  undergoes  when  darkened  is 
interesting  from  a practical  point  of  view  ; and  accordingly 
the  author  made  a series  of  experiments  on  this  subject, 
using  for  that  purpose  coloured  disks  and  the  method  of 
rotation.  In  these  experiments  we  do  not  deal  with  the 
pure  colours  of  the  spectrum,  but  with  surfaces  painted 
with  brilliant  pigments,  which  correspond  more  nearly  to 
the  cases  that  present  themselves  to  the  artist  and  decora- 
tor. A black  disk  was  in  each  case  combined  with  a col- 
oured disk,  as  indicated  in  Fig.  75  ; a smaller  disk  of  the 
same  colour  being  either  attached  to  the  axis  for  compari- 
son or  held  from  time  to  time  near  the  rotating  disk.  It 
was  ascertained  by  previous  experiments  that  the  amount 
of  white  light  reflected  by  the  black  disk  was  small  ; if  we 
set  the  amount  of  light  reflected  by  ^rliite  cardboard  as  100, 


186 


MODERN  CHROMATICS. 


then  the  black  disk  which  was  employed  on  this  occasion 
reflected  two  per  cent,  of  white  light,  or  The  colour  of 
the  painted  disks  was  in  every  case  as  intense,  saturated. 


Fio.  75.— Chrome-yellow  and  Black  Oisk.'i 
in  combination. 


Fig.  76.— The  disk  of  Fig.  75  when  in 
rotation  becomes  coloured  olive-green. 


aiul  brilliant  as  possible.  Tlie  results  obtained  by  rotation 
— that  is,  by  reducing  the  luminosity  of  the  colours  by  mix- 
ing black  with  them — arc  briefly  indicated  below  : 


Table  I. 

Name  of  Colour.  Effect  of  reducing  its  Luminosity. 

Fundamental  red  (carmine  ) i 

, . - ^ot  changed,  or  made  shahilij  purplish, 

andvermdion) \ ° ^ i 

Vermilion More  red,  less  orange-red. 

Red  lead More  red,  less  orange-red. 

Orange Brown. 

Chrome-Yollow  or  gamboge.  Olive-green. 

Greenish-yellow' ilore  greenish. 

Yellowish-green More  pure  green. 

Fuudamental  green Not  changed,  or  made  dighthj  more  bluish. 

Emerald-green More  green,  less  blue-green. 

Blue-green More  green,  less  bluish. 

Cyan-blue More  greenish. 

Prussian-blue Dark  grey-blue  (not  changed). 

Cobalt-blue  . Dark  grey-blue  (not  changed). 

Ultramarine-blue  (artificial).  More  violet,  less  blue. 

Violet Dark  violet. 

Purple.. More  violet,  less  red. 

Carmine Not  much  changed. 


EFFECT  ON  COLOUR  BY  CHANGE  IN  LUMINOSITY.  187 


It  will  be  noticed  that  these  results  correspond  more  or  less 
closely  with  those  of  Von  Bezold,  before  given. 

Some  of  the  changes  in  the  experiments  just  mentioned 
were  so  great  as  to  be  quite  astonishing,  and  might  well 
tempt  the  beholder  to  believe  that  the  black  disk  exercised 
some  peculiar  influence  on  the  result ; this,  however,  was 
not  the  case,  as  the  same  results  can  be  obtained  without 
the  black  disk  by  simply  reducing  the  illumination  of  the 
coloured  disks  by  holding  before  the  eye  two  Mcol’s  prisms, 
and  turning  them  so  as  gradually  to  cut  off  the  coloured 
light.  On  the  other  hand,  if  the  tints  that  are  obtained  by 
using  the  black  disk  give  the  true  appearances  of  surfaces 
painted  with  pure  pigments,  but  viewed  under  a feeble  il- 
lumination, then  accurate  copies  of  them  ought,  when  power- 
fully illuminated,  to  appear  once  more  brightly  coloured, 
and  of  the  original  tints.  This  was  found  to  be  the  case, 
for  example,  with  gamboge,  where  the  change  in  colour  by 
darkening  was  from  a slightly  orange-yellow  to  a fine  olive- 
green.  The  olive-green  colour  was  carefully  copied  with 
water-colours  on  a slip  of  paper,  and  afterward  held  in 
bright  sunlight ; this  caused  it  to  appear  yellow,  and  made 
its  colour  resemble  that  of  the  gamboge  disk  placed  near  it, 
but  in  the  shade. 

The  general  result  of  these  experiments  is,  that,  if  the 
illumination  is  feeble,  the  colours  become  weaker,  and  there 
is  on  the  whole  a general  tendency  toward  a darkish  blue  ; 
just  as  in  the  reverse  case,  where  the  colours  are  made  very 
bright,  there  is  a tendency  toward  a whitish-yellow.  This 
average  tint  can  best  be  studied  by  observing  moonlight 
effects  : here  the  more  luminous  colour  appears  to  be  a 
somewhat  greenish-blue,  the  darker  shades  more  like  an  ultra- 
marine-blue. With  regard  to  this  delicate  point,  the  paint- 
ers of  moonlight  landscapes  are  as  good  an  authority  as  we 
have,  and  the  best  of  them  are  very  decided  as  to  the  prev- 
alence of  various  shades  of  blue,  greenish-blue,  and  violet- 
blue.  Similar  effects,  though  smaller  in  degree,  are  ob- 


188 


MODERN  CHROMATICS. 


served  on  dull,  cloudy  days,  when  the  prevailing  tint  is  a 
bluish-grey.  Indeed,  as  Helmholtz  remarks,  simply  view- 
ing a sunlit  landscape  through  a pale-blue  glass  suggests  the 
klea  of  a cloudy  day  ; while  reversing  the  process,  and 
viewing  a landscape  on  a dull,  cloudy  day  through  a pale- 
yellow  glass,  gives  the  impression  of  sunshine.  Correspond- 
ing to  this,  accidental  streaks  of  yellow  ochre  or  sawdust 
on^the  shaded  pavement  often  suggest  forcibly  He  idea  of 
stray  sunbeams  ; and  other  examples  of  this  kind  of  illu- 
sion might  be  mentioned.  If  we  mix  lampblack  directly 
with  pigments  on  the  palette,  their  colour  will  of  course  be 
darkened,  but  the  effects  produced  are  not  identical  with 
those  obtained  by  the  method  ol  rotation.  Paper  was 
painted  with  a strong  wash  composed  of  caiTnine  and  lamp- 
black, which  imparted  to  it  a dark-reddish,  purplish  hue. 
From  this  a disk  was  cut  and  an  attempt  made  to  match  its 
tint  by  mixing,  according  to  the  method  of  rotation,  car- 
mine and  lampblack.  In  order  to  accomplish  this,  it  was 


Fio. 


white,  ill  the  proportions  indicated. 


round  necessary  to  introduce  into  this  rotation-mixture  a 
ouantitv  of  -n-hite  ; the  best  match  being  effected  when  the 
compound  disk  rvas  arranged  as  indicated  in  Fig.  77.  Th.s 
shows  that  the  saturation  or  intensity  of  a coloured  pigment 
is  oreatlv  reduced  bv  mixing  lampblack  with  it  on  the  pa  - 
ette,  and  is  one  reason  why  artists  refuse  to  adopt  this 


EFFECT  ON  COLOUR  BY  CHANGE  IN  LUMINOSITY.  189 

metliod  of  producing  dark  shades  of  colour.  The  mechani- 
cal mixture  of  lampblack  with  pigments,  besides  reducing 
their  saturation,  also  usually  at  the  same  time  changes  some- 
what their  hue.  In  the  experiment  just  mentioned,  after 
matching  the  two  colours  as  well  as  possible,  it  was  found 
that  the  carmine  which  had  been  mixed  with  lampblack  on 
the  palette  was  more  violet  in  hue  than  that  which  had  been 
mixed  with  black  optically.  Corresponding  results  were 
obtained  with  vermilion  when  mixed  mechanically  and  op- 
tically with  lampblack.  In  the  first  case  the  colour  was 
more  of  an  orange-red  hue  than  in  the  last.  Prussian-blue 
and  lampblack  on  the  palette  give  a much  more  greenish 
tint  than  when  mixed  by  rotation,  and  similar  changes  can 
be  observed  with  many  other  pigments. 

We  have  seen  thus  far  that,  as  we  change  the  luminosity 
of  a coloured  surface,  so  do  we  at  the  same  time  affect  its 
hue,  all  coloured  surfaces  when  very  bright  tending  toward 
a whitish-yellow  tint.  Changes  in  luminosity,  however, 
produce  still  other  effects  which  are  quite  remarkable.  If 
we  arrange  by  ordinary  daylight  sheets  of  red  and  blue  paper, 
which  have  as  far  as  we  can  judge  about  the  same  degree 
of  luminosity,  and  then  carry  them  into  a darkened  room, 
we  shall  be  surprised  to  find  that  the  blue  papers  appear 
brighter  than  the  red.  Indeed,  the  room  may  be  dark- 
ened so  as  to  cause  the  red  paper  to  appear  black,  while 
the  blue  still  plainly  retains  its  colour.  These  facts  seem 
first  to  have  been  recorded  by  Purkinje  and  Dove.  By 
similar  experiments  it  can  be  proved  that  red,  yellow, 
and  orange-coloured  surfaces  are  relatively  more  luminous 
when  exposed  to  a bright  light  than  blue  and  violet  sur- 
faces ; the  latter,  on  the  other  hand,  have  the  advantage 
when  the  illumination  is  feeble.  Thus  Dove  noticed  a long 
time  ago  that  this  circumstance  disturbs  somewhat  the  bal- 
ance of  the  colours  in  paintings,  if  the  observer  lingers  in 
a picture  gallery  as  the  twilight  deepens.  From  this  it 
follows  that  the  chromatic  composition  of  a painting  should 


190 


MODERN  CHROMATICS. 


be  somewhat  varied,  according  as  the  picture  is  likely  gen- 
erally to  be  seen  under  full  daylight  or  in  a darkened  room. 
More  attention  would  no  doubt  be  paid  by  artists  to  this 
point  if  they  were  not  obliged  to  contend  with  a still  more 
serious  obstacle  in  the  large  change  which  the  tint  of  the 
illuminating  light  undergoes,  according  as  daylight  or  gas- 
light is  employed. 

It  follows  from  what  has  just  been  said  that  photomet- 
ric comparisons  of  the  brightness  of  differently  coloured 
surfaces,  if  made  under  bright  daylight,  will  no  longer  hold 
good  in  twilight,  and  that  consequently  we  can  not  under  a 
certain  illumination  establish  photometric  relations  that 
shall  hold  good  under  all  other  illuminations.  For  exam- 
ple, we  may  find  under  ordinary  daylight  that  a certain 
])iece  of  blue  paper  is  just  half  as  luminous  as  a piece  of 
red  paper  ; but  it  by  no  means  follows  that  this  statement 
will  be  true  in  a darkened  room.  Helmholtz  found  that 
even  the  pure  colours  of  the  spectrum  act  in  this  same  man- 
ner, particularly  yellow  and  ultramarine-blue,  or  greenish- 
yellow  and  violet,  the  changes  with  the  other  colours 
being  smaller.  This  fact  suggests  an  interesting  experi- 
ment : Yellow  and  ultramarine-blue  are  complementary, 
that  is,  together  make  up  white  light  ; suppose  now  we 
mingle  yellow  and  ultramarine-blue  so  as  to  produce  white, 
a high  degree  of  illumination  being  employed  ; will  they 
still  produce  white  if  they  are  both  correspondingly  dark- 
ened ? We  might  very  naturally  suppose  that  the  blue 
would  not  outweigh  the  yellow,  and  that  instead  of  white 
we  should  obtain  bluish-white.  This  was,  however,  found 
by  Helmholtz,  using  pure  spectral  colours,  not  to  be  the 
case  : the  darkened  mixture  of  yellow  and  blue  still  exactly 
resembled  sunlight  which  was  correspondingly  darkened. 
The  same  result  was  also  obtained  when  a mixture  of  an- 
other pair  of  complementary  colours,  greenish-yellow  and 
violet,  was  used.  These  results  apparently  contradict  the 
statement  that  yellow  or  greenish-yellow  acts  more  power- 


EFFECT  ON  COLOUR  BY  CHANGE  IN  LUMINOSITY.  191 


fully  on  the  eye  when  bright  than  blue  or  violet.  Helm- 
holtz accounts  for  it  in  this  way  : Our  standard  for  bright 
white  is  bright  sunlight ; we  cause  the  mixture  of  yellow 
and  blue  to  match  this  sunlight,  and  then  call  it  white  ; we 
then  darken  our  sunlight  very  much,  and  make  it  our 
standard  for  a feeble  white  having  a small  degree  of  lumi- 
nosity ; we  call  it  darkened  white  or  pure  grey,  and  find 
that  our  darkened  mixture  of  blue  and  yellow  still  matches 
it  perfectly.  But,  according  to  Helmholtz,  this  pure  grey 
is  really  somewhat  bluish,  and  it  is  owing  to  this  circum- 
stance that  it  is  able  still  to  match  the  darkened  yellow  and 
blue,  which  is  also  really  bluish.  Pure  grey  has  always  ap- 
peared to  the  present  writer  as  somewhat  bluish  compared 
with  pure  white,  and  the  following  experiment  tends  to 
show  that  this  is  indeed  the  case  : A pure  grey  was  gen- 
erated on  a rotating  disk  by  mixing  fifty  parts  of  white 


Fig.  78.— Small  White-and-Black  Disk  arranged  so  as  to  make  a Grey  by  rotation.  This 
prey  looks  more  bluish  than  the  larger  white  disk ; 17  per  cent,  of  an  indigo  disk  Is 
then  mixed  with  the  white. 

with  a like  proportion  of  black.  This  compound  disk  was 
placed  on  the  same  axis  with  a white  disk,  but  when  set  in 
rotation  the  grey  portion  appeared  slightly  more  bluish 
than  the  white.  In  order  to  cause  the  white  disk  to  match 
in  hue  (not  in  luminosity)  the  grey  disk,  a disk  painted 
with  a tolerably  deep  wash  of  indigo  was  combined  with 
the  white  disk,  as  indicated  in  Fig.  78.  An  assistant  ar- 
ranged the  disks  and  made  the  measurements,  while  the 


192 


MODERN  CHROMATICS. 


author  simply  ordered  the  proportion  of  blue  to  be  in- 
creased or  diminished  till  the  result  seemed  satisfactory ; 
at  the  termination  of  the  experiment  he  was  informed  of 
the  result.  The  amounts  of  added  blue  in  nine  consecutive 
experiments  were  as  follows,  in  percentages  : 17,  20,  18,  16, 
13,  14,  21,  19,  16  ; average,  17.  According,  then,  to  these 
experiments,  it  was  necessary  to  add  to  white  17  per  cent, 
of  a strong  tint  of  indigo,  in  order  to  cause  it  to  match  in 
hue  a grey  disk  made  up  of  equal  parts  of  white  and  black. 
It  may  be  remarked  in  this  connection  that  the  addition  of 
the  blue  to  the  white,  although  slightly  changing  its  tint, 
produced  no  particularly  noticeable  effect  on  its  luminosity  ; 
that  is,  it  was  only  a little  darkened. 

All  these  phenomena  can  be  explained  by  the  theory  of 
Young  as  modified  by  Helmhpltz.  According  to  this  the- 
ory, the  sensation  of  white  is  produced  when  the  red, 
green,  and  violet  nerves  of  the  retina  are  stimulated  to 
about  the  same  degree  of  activity  ; furthermore,  with  a fee- 
ble degree  of  stimulation  of  all  three  sets  of  nerves,  the 
activity  of  the  violet  nerves  predominates  over  that  of  the 
green,  and  that  of  the  green  again  over  that  of  the  red. 
When  the  stimulation  is  made  powerful,  these  conditions 
are  reversed,  the  red  nerves  leading,  the  green  and  violet 
following.  These  relations  of  nerve-action  are  indicated 
by  three  curves  in  Fig.  79.  Tlie  horizontal  line  represents 
increase  of  actual  intensity  of  white  light ; thus  the  portion 
A B stands  for  feeble  white  light,  A C for  white  light  which 
is  twice  as  strong,  etc.  The  vertical  line  B R measures  the 
intensity  of  the  red  sensation  produced  by  this  feeble  white 
light  ; B G and  B V give  the  strength  of  the  sensations  in 
the  case  of  the  green  and  violet  nerves.  W e see  that  the 
violet  sensation,  as  it  is  represented  by  the  longest  line, 
prevails  over  the  others,  and  that  the  light,  instead  of  ap- 
pearing white,  will  be  such  as  would  be  produced  by  mix- 
ing equal  parts  of  the  sensations  red,  green,  and  violet  ; 
i.  e.,  by  mixing  the  sensation  of  white  with  a little  green 


EFFECT  ON  COLOUR  BY  CHANGE  IN  LUMINOSITY.  193 

and  a little  more  violet.  I^ow,  as  we  have  previously  seen 
in  Chapter  IX.,  the  sensations  of  green  and  violet  when 
mixed  produce  that  of  blue,  so  that  on  the  whole  we  have  a 
mixture  of  the  sensation  of  pure  white  with  blue,  i.  e.,  blu- 
ish-white. If  we  examine  in  the  same  way  the  line  D V G R, 
we  shall  find  the  conditions  reversed  ; here  we  have  the 
sensation  of  pure  white  mixed  with  a slight  excess  of  that 
of  green,  and  again  with  a little  more  of  red  ; as  green  and 


F i(j.  79. — Three  Curves  showing  the  action  of  the  Eed,  Green,  and  Violet  Nerves  when 
stimulated  by  White  Light  of  different  degrees  of  Brightness. 


red  when  mixed  furnish  yellow,  our  result  now  will  be 
white  mixed  with  yellow,  or  yellowish-white.  This  same 
diagram  also  represents  in  a symbolic  manner  the  fact  that 
red  surfaces  are  most  luminous  by  bright  light,  and  violet 
surfaces  by  feeble  light.  It  can  also  be  used  to  explain 
the  changes  which  pure  colour  undergoes  when  made  very 
bright  or  very  pale,  after  the  manner  employed  in  the  early 
part  of  the  present  chapter. 

We  have  examined  now  with  some  detail  the  relative 
changes  in  luminosity  which  coloured  surfaces  undergo 
when  exposed  to  bright  and  feeble  illuminations  ; but,  be- 
fore leaving  this  part  of  our  subject,  it  may  not  be  amiss  to 
mention  the  fact  that  all  comparisons  between  the  luminosi- 
ties of  differently  coloured  surfaces  are  quite  valueless  as 
the  expression  of  objective  facts.  An  illustration  will  make 


104 


MODERN  CHROMATICS. 


this  clear.  Suppose  we  compare  together  by  the  eye  two 
white  surfaces  or  two  red  surfaces,  and  find  that  they  ap- 
pear to  us  equally  luminous  ; now,  this  will  be  not  only  a 
fact  as  far  as  the  eye  is  concerned,  but  it  will  be  an  objec- 
tive fact  in  nature  ; it  will  be  equally  true  in  a mechanical 
sense,  and  on,  converting  our  two  masses  of  white  light  or 
red  light  into  heat,  we  shall  obtain  equal  amounts  of  heat, 
tf,  however,  we  take  two  differently  coloured  surfaces,  red 
and  blue  for  example,  and  make  them  equally  luminous, 
equally  powerful  so  far  as  the  eye  is  concerned,  and  then 
convert  the  light  they  reflect  into  heat,  a delicate  thermo- 
metric apparatus  will  speedily  inform  us  that  we  are  not 
dealing  in  the  two  cases  with  equal  amounts  of  force.  In 
fact,  the  maximum  heating  effect  was  found  by  Melloni  to 
be  produced  by  the  ultra-red  rays  which  are  quite  invisible 
to  the  eye  ; here,  from  an  objective  point  of  view,  resides 
the  greatest  force,  but  the  waves  are  too  long  to  affect  the 
eye  at  all.  From  this  it  is  evident  that  the  intensity  of  our 
visual  sensations  depends  not  only  on  the  strength  (height 
or  amplitude)  of  the  waves  of  light,  but  also  on  their 
length  ; the  maximum  effects  being  produced  by  yellow 
light,  which  affects  simultaneously  the  red  and  green  nerves. 
In  spite  of  the  fact  that  photometric  comparisons  of  differ- 
ently coloured  surfaces  have  no  objective  value,  still  for  our 
purposes  they  may  often  be  quite  precious,  or  even  actually 
indispensable,  if  we  propose  to  give  our  work  a quantitative 
character. 

Having  now  considered  the  changes  which  colour  under- 
goes when  made  very  luminous  or  very  feeble,  we  proceed 
to  study  the  effects  produced  by  mingling  white  with  it. 
The  general  result  can  be  expressed  quite  concisely  : the 
colour  becomes  paler,  and  when  the  proportion  of  white  is 
made  large  entirely  disappears,  leaving  recognizable  only  a 
white  surface.  When  a disk  painted  as  in  Fig.  80  is  ro- 
tated, the  red  by  mixture  with  the  white  gives  a ring  of 
pale  red,  like  that  indicated  in  Fig.  81.  F pon  reducing  the 


EFFECT  ON  COLOUR  BY  CHANGE  IN  LUMINOSITY.  195 


amount  of  tlie  colour,  it  becomes  very  whitish  and  pale  ; 
and  Aubert  found  that  the  red  when  mixed  with  from  120 
to  180  parts  of  white  entirely  disappeared.  If  we  set  the 
luminosity  of  vermilion  as  one  fourth  of  that  of  white  paper, 
it  follows  from  Aubert’s  experiment  that  mixing  vermilion 


with  720  parts  of  white  light,  having  a brightness  equal  to 
its  own,  causes  the  red  colour  to  disappear.  Or  we  may 
express  the  fact  thus  : Take  red  light  and  white  light  of 
equal  intensities  ; then,  if  one  part  of  red  light  be  presented 
to  the  eye  simultaneously  with  720  parts  of  white  light,  the 
eye  wdll  be  unable  to  recognize  the  presence  of  the  red  con- 
stituent. Smaller  quantities  of  white  light  produce  very 
great  changes  in  the  appearance  of  the  colour.  If  we  rotate 
a disk  like  that  indicated  in  Fig.  79,  we  shall  be  surprised 
to  find  that,  though  one  quarter  of  the  disk  is  covered  with 
vermilion,  yet  the  resultant  red  tint  is  quite  pale.  In  this 
case  twelve  parts  of  white  light  are  mixed  with  one  part  of 
equally  bright  red  light,  and  when  stated  in  this  manner 
the  result  seems  more  natural. 

When  we  undertake  to  study  more  carefully  the  mix- 
tures of  white  with  coloured  light,  certain  curious  anoma- 
lies present  themselves.  If  we  arrange  disks  of  artificial 
iiltramarinc-blue  and  white,  in  the  >vay  shown  in  Fig.  82, 


Fig.  80.— White  Disk  partially 
painted  Eed  with  Vermilion. 


Fig.  81. — Indicates  the  appear- 
ance presented  by  the  previ- 
ous disk  when  set  in  rapid 
rotation  so  as  to  mix  the  red 
and  white  light. 


196 


MODERN  CHROMATICS. 


and  set  them  in  rapid  rotation,  we  shall  find  that  the  addi- 
tion of  white,  instead  of  producing  merely  a paler  blue. 


Fio.  62.— White  Cardboard  Disk 
partially  painted  with  Ultra- 
marine-blue (artificial). 


Fig.  83.— Indicates  the  appear- 
. ance  produced  by  mixing  white 
with  ultramarine-blue  light. 


actually  changes  the  colour  to  a pale  violet.  (See  Fig.  83.) 
If  we  substitute  orange  for  the  ultramarine-blue,  the  pale- 
orange  hue  generated  by  rotation  shows  a tendency  tow^ard 
red.  These  tw^o  facts  have  been  known  for  a long  time, 
and  various  explanations  of  them  have  been  proposed. 
According  to  Briicke,  ordinary  white  daylight  is  itself 
slightly  reddish  in  tint,  and,  when  we  mix  white  light  with 
coloured  light,  we  really  add  at  the  same  time  a little  red  ; 
hence  these  changes.  Aubert,  on  the  other  hand,  following 
a suggestion  of  Helmholtz,  supposes  that  the  true  pale  shade 
of  ultramarine-blue  is  actually  violet,  *but  that  our  judg- 
ment is  perverted  by  experience  drawn  from  the  colour  of 
the  sky,  wdiich  according  to  him  is  a greenish-blue,  and  re- 
tains this  tint  when  mixed  with  ’svhite.  This  is  an  explana- 
tion which  artists  would  hardly  accept,  and  the  experi- 
ments given  below  show'  that  both  it  and  the  one  previously 
cited  are  insufficient.  In  an  examination  of  this  matter  it 
was  found  by  the  author  that  changes  of  this  kind  are  not 
confined  to  the  colours  orange  and  artificial  ultramarine- 
blue,  but  extend  over  all  the  colours  except  violet  and  its 
complement  greenish-yellow  ; the  main  results  are  given  in 
the  following  table  ; 


EFFECT  ON  COLOUR  BY  CHANGE  IN  LUMINOSITY.  197 


Table  II. — Showing  the  Effects  of  mixing  White  with  Coloured 

Light, 

Name  of  Colour, 

Vermilion.  

Orange 

Chrome-yellow 

Pure  yellow 

Greenish-yellow 

Green 

Emerald-green 

Cyan-blue 

Cobalt-blue 

Ultramarine  (artificial). 

Violet. 

Purple 

It  follows  from  these  experiments  that,  when  we  mix  white 
with  coloured  light,  the  effect  produced  is  the  same  as 
though  we  at  the  same  time  mixed  with  our  white  light 
a small  quantity  of  violet  light.  Such  mixture  would  ac- 
count for  all  the  changes  given  in  the  table,  as  could  be 
shown  by  reference  to  the  colour-diagram  explained  in  the 
next  chapter.  This,  of  course,  is  only  stating  the  facts  in 
different  language,  and  is  not  an  explanation. 

In  the  experiments  just  mentioned  the  light  from  bril- 
liantly coloured  disks  was  mixed  by  rotation  with  from  5 
to  50  per  cent,  of  white  light,  some  of  the  disks  requiring  a 
larger  admixture  of  white  light  than  others  to  produce  the 
changes  in  hue  recorded  in  the  table.  If  now  we  first 
darken  our  colours  very  much,  and  then  add  a little  white 
to  them,  the  results  will  again  be  somewhat  different  from 
those  given  in  the  table,  because  here  two  causes  are  at 
work,  which  sometimes  produce  opposite  results,  as  we  can 
see  by  a comparison  of  Tables  I.  and  II.  In  the  next  set  of 
experiments,  in  every  case  except  that  of  chrome-yellow,  5 
parts  of  the  coloured  disk  were  combined  with  90  parts  of 
pure  black  and  5 parts  of  white  ; 10  parts  of  chrome-yel- 


Eflfect  of  adding  White. 
More  purplish. 

More  red. 

More  orauge-yellow. 
More  orange-yellow. 
Paler  (unchanged). 
More  blue-green. 
More  blue-green. 
More  bluish. 

A little  more  violet. 
More  violet. 
Unchanged. 

Less  red,  more  violet. 


198 


MODERN  CHROMATICS. 


low  were  combined  with  85  parts  of  pure  black  and  5 of 
white.  The  changes  of  hue  are  given  below  : 

Table  III. — Showing  the  Effect  of  making  Colour  very  Dark  and  add- 
ing TO  IT  A SMALL  PORTION  OF  WHITE.  (The  experiments  were  made  by 

rotating  disks.) 

Vermilion  became  a Dull  greyish-purple. 

Orange  became  a Brown  (slightly  bluish). 

Chrome-yellow  became  a Greyish  olive-green. 

Emerald-green  became  a Dark  green  (less  bluish). 

Cyan-blue  became  a Dark  greenish-grey. 

Prussian-blue  became  a Dark  grey-blue. 

Cobalt-blue  became  a Dark  grey-blue. 

Ultramarine  (artificial)  became  a Dark  grey  violet-blue. 

Violet  became  a Dark  grey-violet. 

Purple  became  a Dark  grey-violet  (less  red). 

In  many  of  these  cases  the  results  are  similar  in  charac- 
ter to  those  given  in  Table  II.  This,  however,  is  not  the 
case  with  chrome-yellow,  as  in  one  case  it  was  made  to 
appear  more  orange,  while  in  the  other  it  became  a whitish 
olive-a'reen.  It  is  evident  that  in  this  instance  the  elfect  of 
darkening  the  colour  overbalanced  that  of  adding  white  to 
it.  With  emerald-green  and  cyan-blue  a similar  result 
seems  to  have  been  reached,  though  the  phenomena  were 
less  decided.  The  general  effect,  then,  of  first  reducing 
greatly  the  luminosity  of  colour  and  then  adding  small 
amounts  of  white,  is  the  production  of  greys  which  have  a 
tendency  toward  blue  or  violet,  this  being  the  case  even 
when  the  original  colour  is  as  decided  as  that  of  vermilion. 
The  experiments  given  in  the  last  two  tables  will  account 
for  the  fact  that  it  is  almost  impossible  to  produce  a fine 
red  with  the  aid  of  polarized  light,  the  tint  being  always 
rather  of  a rose-colour,  that  is,  showing  a tendency  toward 
a purplish  hue. 

It  has  been  shown  in  the  preceding  pages  that  the  effect 
of  mixins:  white  with  coloured  light  is  to  cause  the  colour 
to  become  paler,  and  at  the  same  time  to  change  it  slightly. 


EFFECT  ON  COLOUR  BY  CHANGE  IN  LUMINOSITY.  199 


as  though  simultaneously  a small  amount  of  violet  light 
had  been  added  to  the  mixture.  This  fact  naturally  sug- 
gests an  experiment  like  the  following  : Suppose  we  com- 
bine a purple  and  a green  disk  as  indicated  in  Fig.  84, 
employing  equal  parts,  and  thus  obtain  a pure  grey.  Let 


Fig.  84. — Purple  and  Green  Disk  : 
when  rotated,  it  makes  a Pure 
Grey. 


Fig.  86.— Purple,  Green,  and  White 
Disk : when  rotated,  it  makes  a 
Pure  Grey. 


US  now  replace  10  parts  of  purple  by  white,  also  10  parts  of 
the  green  by  white  (Fig.  85)  : will  we  then  still  obtain  a 
pure  grey,  or  will  the  grey  be  tinged  with  violet  ? Several 
experiments  of  this  kind  have  been  accurately  made  by  the 
author,  but  in  every  case  the  result  was  the  production  of  a 
grey  identical  with  that  given  by  mixing  by  rotation  black 
and  white.  The  explanation  would  seem  to  be  that  the 
green  and  purple  instantly  combine  to  produce  the  sensa- 
tion of  grey,  and  then  of  course  adding  white  to  this  grey 
can  only  make  it  paler,  but  can  not  at  all  alter  its  tint.  It 
would  seem  from  this  that,  when  a colour  is  altered  in  the 
manner  above  described  by  admixture  with  white,  time 
comes  in  as  a necessary  element  in  the  process  ; the  mixture 
of  white  and  coloured  light  must  be  allowed  to  act  on  the 
eye  undisturbed  during  an  interval  of  time  which  is  not  too 
short,  otherwise  these  peculiar  effects  will  not  be  produced. 


One  might  suppose  that  the  same  result  would  be  pro- 
duced by  spreading  thin  washes  of  coloured  pigments  on 


200 


MODERN  CHROMATICS, 


white  paper  that  is  obtained  by  mixing  white  with  coloured 
pigments  by  the  method  of  revolving  disks.  The  tint  in 
these  two  cases,  however,  is  usually  somewhat  different. 
A pale  wash  of  carmine,  for  example,  was  allowed  to  dry 
on  white  paper,  and  an  effort  was  made  to  imitate  it  by 
combining  a deep-coloured  carmine  disk  with  one  of  white 
cardboard  by  the  method  of  rotation.  It  was  soon  ascer- 
tained that  the  hue  of  the  water-colour  wash  was  consider- 
ably more  saturated  or  intense  than  a tint  of  equal  luminos- 
ity produced  by  the  rotating  disks  ; it  was  also  found  to 
be  more  of  a purplish  hue.  When  the  luminosities  in  the 
two  cases  were  made  equal,  the  water-colour  wash  showed 
far  more  colour  than  did  the  simple  mixture  of  the  red  and 
white  light.  Treated  in  the  same  way,  a thin  wash  of  ver- 
milion was  more  orange  in  hue  than  a mixture  of  vermilion- 
coloured  light  with  white  light ; a thin  wash  of  gamboge 
looked  yellow,  while  the  mixture  by  rotation  had  more  of 
an  orange-yellow  appearance.  The  reason  of  these  changes 
is  quite  evident,  and  lies  in  the  well-known  fact  that  thin 
layers  of  coloured  substances  have  in  general  a different 
absorptive  action  on  white  light  from  thicker  layers  of  the 
same  substances.  A thin  layer  of  vermilion  allows,  for  ex- 
ample, more  of  the  orange  rays  to  pass  ; hence  in  very  thin 
layers  this  pigment  is  sometimes  used  by  artists  to  represent 
very  pale  tints  of  orange,  or  even  of  orange-yellow.  The 
other  fact  above  mentioned,  viz.,  that  thin  layers  are  often 
relatively  more  saturated  than  those  that  are  thick,  is  to  be 
explained  in  a different  way.  It  was  shown  in  Chapter  X. 
that,  when  a pigment  is  mixed  with  one  of  a different  col- 
our, not  only  is  the  hue  changed,  but  an  effect  is  produced 
as  though  at  the  same  time  some  black  had  been  added  to 
the  mixture.  It  now  appears  that,  even  when  a pigment  is 
made  darker  by  mixing  with  it  a larger  quantity  of  itself, 
a similar  change  is  to  some  extent  produced,  the  darker 
wash  of  the  pure  pigment  acting  as  though  some  black  were 
mingled  with  it.  In  the  experiment  with  the  pale  wash  of 


EFFECT  ON  COLOUR  BY  CHANGE  IN  LUMINOSITY.  201 


carmine  it  was  actually  found  necessary  to  combine  black 
by  rotation  with  the  water-colour  wash,  so  as  to  reduce  it, 
before  it  could  be  matched  by  a disk  composed  of  white 
and  a deep  tint  of  carmine. 

The  fact  now  under  consideration  can  perhaps  be  ren- 
dered more  intelligible  by  a different  statement.  Carmine, 
as  we  know,  absorbs  powerfully  nearly  all  the  coloured  rays 
of  light  except  the  red  ; thesd  latter  it  reflects  in  considera- 
ble quantity,  and  to  this  circumstance  its  red  colour  is  due. 
But  the  experiments  just  mentioned  indicate  that  it  absorbs 
also  to  a considerable  extent  even  the  red  rays,  so  that  a 
deep  wash  of  carmine  sends  to  the  eye  less  red  light  than 
we  should  expect.  The  author  found  that  yellow  glass  pre- 
sented a parallel  case.  Yellow  glass  transmits  the  orange- 
yellow,  yellow,  and  greenish-yellow  rays  abundantly,  and 
to  this  power  mainly  its  yellow  colour  is  due.  But  it  does 
not  transmit  even  these  rays  at  all  as  perfectly  as  ordinary 
window-glass.  In  one  experiment  it  was  found  that  a plate 
of  yellow  glass  absorbed  about  25  per  cent,  even  of  these 
rays.  Most  coloured  substances,  pigments,  and  glasses 
probably  act  in  a similar  way. 


CHAPTER  XIII. 


ON  THE  DURATION  OF  THE  IMPRESSION  ON  THE 
RETINA. 

Among  different  forms  of  fireworks  none  excite  more 
admiration  than  revolving  wheels  of  fire  with  their  brilliant 
colours,  ruby,  emerald,  or  sapphire,  and  their  wonderfully 
blended  surfaces,  which  so  often  suggest  fanciful  resem- 
blances to  roses,  carnations,  and  other  flowers.  It  is  quite 
possible  to  arrange  matters  so  as  to  obtain  an  instantaneous 
view  of  one  of  these  fiery  objects,  Avithout  at  all  interfering 
with  its  rapid  movement  ; and  when  this  is  done,  it  is  seen 
that  much  of  its  beauty  depends  upon  an  illusion  : the 
broad,  variegated,  shaded  surface  vanishes,  and  Ave  haA^e 
before  us  simply  a feAV  jets  of  coloured  fire,  in  no  wise  par- 
ticularly remarkable.  The  appearance  of  these  brilliant 
objects  depends,  then,  upon  an  illusion,  and  this  has  for  its 
foundation  the  fact  that  the  sensation  of  sight  is  always 
prolonged  after  the  light  producing  it  has  ceased  to  act  on 
the  eye. 

The  most  familiar  illustration  of  this  fact  Ave  find  in  an 
old  experiment,  which  no  doubt  was  the  parent  of  our  re- 
volving fireworks  : If  a lighted  coal  on  the  end  of  a stick 
is  caused  to  revoh^e  rapidly,  it  describes  a ring  of  fire  which 
is  plainly  seen  at  night  to  be  quite  unbroken.  The  light 
from  the  moAung  coal  falls  upon  the  retina  of  the  eye,  and 
an  image  of  it  is  produced,  let  us  say  at  the  point  1,  Fig. 
86  ; an  instant  afterward,  OAAung  to  its  having  moA^ed  into  a 
new  position,  the  image  Avill  be  found  at  2 and  then  at  3, 


DURATION  OF  THE  IMPRESSION  ON  THE  RETINA.  203 


and  so  on  all  the  way  around  the  circle.  Now,  if  the  sen- 
sation due  to  the  first  image  lasts  while  the  circle  is  being 
traversed,  then  it  will  be  renewed  before  it  has  a chance  to 
fade  out,  and  consequently  will  be  present  continuously  ; 
the  same  will  be  true  of  all  other  points  on  the  circle,  which 
consequently  will  produce  on  the  beholder  the  appearance 
of  an  unbroken  ring  of  light.  In  order  to  produce  this 
condition  of  things,  it  is  necessary  of  course  that  the  coal 
of  fire  should  move  with  a certain  velocity  ; according  to 


Fig,  sc.— Appearance  of  a Coal  of  Fire  in  Three  Positions. 


the  observations  of  D’Arcy,  it  is  essential  that  it  should 
traverse  its  circular  path  completely  in  thirteen  hundredths 
of  a second. 

It  is  very  easy  to  experiment  on  matters  like  these  with 
the  aid  of  revolving  disks.  If  we  take  a circular  disk 
which  is  painted  black,  and  has  on  it  a white  spot  like  that 
represented  in  Fig.  87,  and  set  it  in  rotation,  as  soon  as  the 
motion  is  quick  enough  we  shall  see  a ring  of  white,  just  as 
in  the  previous  case  we  obtained  a ring  of  fire.  Fig.  88 
shows  the  appearance  of  the  disk  when  in  rapid  rotation. 


204 


MODERN  CHROMATICS. 


The  duration  of  the  sensation  of  light,  or  duration  of  the 
impression  on  the  retina,  as  it  is  called,  varies  with  the  in- 
tensity of  the  light  producing  it,  and  in  the  case  of  our 
white  paper  is  not  by  any  means  so  great  as  with  the  coal 
of  fire.  According  to  an  experiment  of  Helmholtz,  the  im- 
pression on  the  retina  lasts  in  this  case,  with  undiminished 
strength,  about  one  forty-eighth  of -a  second;  hence  it  is 
necessary  for  the  disk  to  revolve  forty-eight  times  in  a sec- 
ond in  order  to  produce  the  appearance  of  a steady,  uniform 
ring  of  light.  While,  as  just  stated,  the  impression  lasts 
with  undiminished  strength  for  one  forty-eighth  of  a sec- 
ond, its  total  duration  with  decreasing  strength  is  much 
greater,  being  perhaps  as  high  as  one  third  of  a second, 
though  this  interval  varies  somewhat  with  the  circum- 
stances, and  is  a little  diflicult  of  determination. 


Fig.  87.— Disk  with  White  Sector.  Fig.  88.— Disk  of  Fig.  87  In  Rapid 

Rotation. 


It  is  not,  however,  to  be  supposed  that,  in  the  experi- 
ment indicated  in  Fig.  88,  the  ring  of  white  light  will  have 
the  same  degree  of  luminosity  as  its  source,  viz.,  the  slip  of 
white  paper  pasted  on  the  black  disk  ; on  the  contrary,  the 
luminosity  of  the  ring  will  be  much  feebler  than  that  of  the 
spot.  The  reason  of  this  is  quite  evident  : we  have  virtu- 
ally spread  out  the  light  of  the  sjDot  over  a much  larger 
surface,  and  it  will  be  proportionately  weaker  ; if  the  sur- 
face of  the  ring  is  one  hundred  times  as  great  as  that  of 
the  spot,  then  the  luminosity  of  the  ring  will  be  exactly  one 
hundredth  of  that  of  the  spot.*  That  this  relation  is  always 


DURATION  OF  THE  IMPRESSION  ON  THE  RETINA.  205 

strictly  quantitative  can  be  proved  by  a photometric  ar- 
rangement like  that  used  by  Plateau  for  this  purpose,  or  by 
the  aid  of  a crystal  of  calc  spar  which  divides  ordinary  light 
up  into  two  beams  of  equal  intensity.  In  this  latter  case 
we  arrange  a disk  by  making  half  of  its  surface  white,  the 
other  half  black,  as  represented  in  Fig.  89  ; alongside  of  it, 
on  a black  ground,  we  place  a strip  of  the  same  white  pa- 
per ; the  disk  is  then  set  in  fetation,  and  assumes  a grey 
appearance.  With  the  aid  of  the  calc-spar  prism  we  then 
view  the  strip  of  paper,  which  will  appear  doubled,  as  shown 
in  Fig.  90,  each  image  having  one  half  the  brightness  of  the 


90.— Appearance  of  Disk  in  Eota- 
Strip  for  Photometric  Comparison.  tion.  the  Strip  being  viewed  through 

a Calc-Spar  Prism. 


original  strip  ; but  either  of  these  images  will  be  as  bright 
as  the  grey  disk,  showing  that  the  luminosity  of  the  grey 
disk  is  just  one  half  of  that  of  the  white.  This  gives  an 
idea  of  the  method  of  proceeding,  but  numerous  corrections 
must  be  introduced,  some  of  which  will  no  doubt  suggest 
themselves  to  the  ingenious  reader  ; of  them  we  will  men- 
tion only  one,  viz.  : that,  according  to  the  recent  investiga- 
tions of  W^ild,the  two  images  furnished  by  calc  spar  do  not 
really  have  exactly  the  same  luminosity,  but  differ  by  about 
three  per  cent.*  Dove  has  proved  that  the  relation  we 
liave  been  speaking  of  above  also  holds  good  for  coloured 


* Poggendorff’s  “ Annalen,”  vol.  cxviii.,  p.  225. 


206 


MODERN  CHROMATICS. 


light.  This  fact  is  of  much  importance  to  us,  for  on  it  is 
based  the  principle  involved  in  Maxwell’s  disks,  which  we 
have  already  found  so  indispensable  in  quantitative  investi- 
gations on  colour. 

The  duration  of  the  impression  on  the  retina  in  the  case 
of  light  of  different  colours  has  not  yet  been  studied  care- 
fully. Some  experiments  were  made  by  Plateau  with  pa- 
pers coloured  by  gamboge,  carmine,  and  Prussian-blue,  and 
it  was  ascertained  that  in  these  cases  the  duration  differed 
somewhat.  Dr.  Wolcott  Gibbs  suggested  to  the  author  a 
method  which  would  probably  solve  this  problem  in  a satis- 
factory manner,  and  '^^'hich  is  about  as  follows  : With  the 
aid  of  a spectroscope  a diffraction  spectrum  is  to  be  pre- 
sented to  the  eye  in  the  form  of  a series  of  contiguous  col- 
oured bands,  this  division  into  bands  being  effected  by  a 
suitable  diaphragm  placed  in  the  eyepiece  of  the  instru- 
ment. In  front  of  the  slit  of  the  spectroscope  a revolving 
disk  with  one  or  more  openings  should  allow  the  light  to 
enter  the  instrument  ; and,  by  regulating  carefully  the  ve- 
locity of  rotation,  it  would  be  possible  to  seize  the  exact 
moment  when  one  or  more  of  the  coloured  bands  ceased  to 
flicker,  and  presented  a steady,  uniform  appearance.  This 
observation  would  give  correctly  the  interval  during  which 
the  impression  remained  with  undiminished  strength  on  the 
eye,  in  the  case  of  the  selected  colour. 

If  quite  bright  light  like  that  from  a window  is  present- 
ed to  the  eye,  the  impression  lasts  several  seconds — of  course 
with  diminishing  strength.  The  experiment  is  easily  made, 
and  the  observer  will  find,  after  the  eyes  have  been  closed, 
that  there  is  time  enough  to  recognize  a good  many  details 
before  the  disappearance  of  the  image.  With  intensely 
bright  light  like  that  of  the  sun,  the  image  lasts  several 
minutes,  and  finally  fades  out  after  having  undergone  a se- 
ries of  complicated  changes  in  colour.  It  may  perhaps  be 
as  well  to  mention  here  a fact  which  must  be  borne  in  mind 
when  we  undertake  to  study  the  after-sensations  that  follow 


DURATION  OF  THE  IMPRESSION  ON  THE  RETINA.  207 


the  action  of  white  or  coloured  light  on  the  eye.  If  the 
light  be  allowed  to  act  for  a short  time  on  the  eye,  when  it 
vanishes,  as  before  stated,  the  sensation  remains  for  a frac- 
tion of  a second,  this  after-sensation  being  in  all  respects 
identical  with  the  original  sensation,  except  that  it  gradual- 
ly becomes  weaker  and  weaker  : thus,  if  the  original  sensa- 
tion is  red,  the  after-sensatioii  will  entirely  correspond  to 
this  colour.  This  after-image,  which  is  the  only  one  thus 
far  treated  of  in  this  chapter,  is  called  the  positive  image. 
After  a little  while,  however,  the  positive  image  vanishes, 
and  is  replaced  by  an  image  of  a different  character,  which 
is  known  as  the  negative  image  : thus,  if  the  light  original- 
ly acting  on  the  eye  was  red,  the  negative  image  is  coloured 
greenish-blue,  or  has  the  complementary  colour.  These 
negative  images  are  quite  important,  as  many  matters  con- 
nected with  contrast  depend  on  them,  and  they  will  be  con- 
sidered at  length  in  Chapter  XV. 

We  have  seen  that  the  positive  after-images  are  useful 
in  furnishing  us,  in  the  case  of  revolving  disks,  with  a mode 
of  mixing  together  masses  of  coloured  light  in  definite  pro- 
portions ; and  it  may  be  well  to  mention  some  other  cases 
where  these  images  play  an  important  part.  The  appear- 
ances presented  by  water  in  motion  depend  largely  on  them. 
Thus,  if  we  study  the  ocean  waves  under  direct  sunlight,  we 
shall  find  that  much  of  their  character  depends  on  elongated 
streaks  of  light,  which  serve  to  interpret  not  only  the  forms 
of  the  larger  masses  of  water,  but  also  the  shapes  of  the 
minor  wavelets  with  which  these  are  sculptured.  If  now 
we  examine  these  bright  streaks,  so  well  known  to  artists, 
with  a slowly  revolving  disk  having  one  open  sector,  we 
shall  find  that  in  point  of  fact  there  are  no  streaks  at  all 
present,  but  simply  round  images  of  the  sun,  which,  owing 
to  their  motion,  become  thus  elongated.  Instantaneous 
photographs  give  the  same  true  result,  and  hence  appear 
false.  An  analogous  action  takes  place  even  in  cloudy 
weather,  and  streaks  of  light  are  produced  which  give  the 


208 


MODERN  CHROMATICS. 


waves  a different  appearance  from  what  they  would  have  if 
suddenly  made  solid,  while  yet  retaining  all  their  glassy 
appearance.  Again,  for  the  same  reason,  waves  breaking 
on  a beach  appear  to  us  different  from  their  instantaneous 
photographs  : when  viewing  the  real  waves  we  obtain  an 
impression  which  is  made  up  of  the  different  views  rapidly 
presented  during  several  minute  intervals  of  time,  whereas 
the  photograph  gives  us  only  what  takes  place  during  one 
of  these  small  intervals.  All  this  applies  also  more  or  less 
to  the  case  of  falling  water,  as  fountains  or  waterfalls,  and 
explains  the  transparency  of  rapidly  revolving  wheels. 
Owing  to  the  same  cause,  the  limbs  of  animals  in  swift 
motion  are  only  visible  in  a periodic  way,  or  at  those  mo- 
ments when  their  motion  is  being  reversed  ; during  the  rest 
of  the  time  they  are  practically  invisible.  These  moments 
of  comparative  rest  are  seized  by  artists  for  delineation, 
while  the  less  discriminating  photograph  is  as  apt  to  repro- 
duce intermediate  positions,  and  thus  produce  an  effect 
which,  even  if  quite  faithful,  still  appears  absurd.* 

* For  a complete  list  of  what  has  been  published  on  this  whole  subject, 
see  a memoir  by  J.  Plateau  published  by  the  Belgian  Academy  of  Sciences 
in  1811. 


CHx\PTER  XIV. 


ON  THE  MODES  OF  ARRANGING  COLOURS  IN  SYSTEMS. 

As  we  have  seen  in  the  previous  chai3ters,  the  variety  of 
colours  presented  to  us  by  nature  and  art  is  enormous,  rang- 
ing from  the  pure  brilliant  colours  of  the  spectrum  down  to 
the  dull,  pale  tints  of  rocks  and  earth,  and  including  whole 
classes  which  seem  at  first  glance  to  have  but  little  affinity 
with  each  other.  It  would  be  difficult,  for  example,  to  find 
anything  in  the  prismatic  spectrum  which  reminds  us  in  the 
least  of  the  colour  brown ; the  various  pale  tints  which 
wood  assumes  when  worked  seem  quite  unrelated  to  the 
spectral  hues  ; and  it  is  the  same  with  the  vast  multitude 
of  greys  with  which  we  are  acquainted,  and  which  so  large- 
ly constitute  the  colours  of  natural  scenery.  If,  instead  of 
comparing  with  the  colours  of  the  spectrum  the  strange, 
wonderful,  indescribable  tints  which  nature  so  abundantly 
displays,  we  descend  a step  and  think  of  them  in  connection 
with  the  hues  furnished  by  our  brightest  pigments,  such  as 
vermilion,  red  lead,  chrome-yellow,  emerald-green,  or  ultra- 
marine-blue, we  scarcely  improve  the  matter : even  such 
colours  as  these  seem  to  have  no  affinities  with  the  modest 
throng  of  greys  and  browns  ; they  appear  to  belong  to  a 
more  princely  caste,  and  utterly  refuse  to  mingle  on  equal 
terms  with  the  humbler  multitude. 

The  colour  of  vermilion  depends,  as  we  learned  some 
time  ago,  on  three  things  : first,  on  the  quantity  of  coloured 
light  which  it  sends  to  the  eye,  or  on  the  brightness  of  tlie 


210 


MODERN  CHROMATICS. 


coloured  light  that  it  reflects  ; second,  on  the  wave-length 
of  this  red  light ; and  third,  on  the  amount  of  loliite  light 
which  is  mingled  with  the  red.  Its  colour  depends,  then, 
on  its  luminosity,  wave-length,  and  purity  ; these  quanti- 
ties, as  we  have  seen,  are  called  the  constants  of  colour.* 
In  certain  cases  we  can  easily  alter  these  constants  consider- 
ably, and  thus  ascertain  their  significance  with  regard  »to 
colour.  Let  us  take  a circular  disk  painted  with  pure  ver- 
milion, and  undertake  to  reduce  its  luminosity.  This  could 
be  very  efficiently  accomplished  by  removing  the  disk  to  a 
darkened  room,  wdien  it  would  be  found  that  the  colour  was 
changed  to  dark  red,  or  even  to  black.  There  is,  however, 
an  objection  to  this  mode  of  experimenting  : Ave  have  al- 
ways, under  such  circumstances,  a strong  tendency  not  sim- 
ply to  receive  the  tint  that  is  actually  presented  to  us,  but 
to  make,  unconsciously,  an  allowance  for  the  degree  of  il- 
lumination under  which  we  view  it.  AV e know  that  red  in 
a darkened  room  presents  a certain  appearance  ; Avhen  Ave 
see  this  appearance  in  the  darkened  room,  we  say  Ave  see 
dark  red  ; to  the  same  appearance  in  a light  room  we  Avould 
give  a different  name.  This  objection  applies  to  all  experi- 
ments of  this  character,  Avhether  the  object  be  to  expose  a 
surface  to  a very  small  or  to  a very  great  illumination  : we 
oursehms  should,  as  it  Avere,  ahvays  be  immersed  in  a medi- 
um illumination,  so  as  to  retain  an  undisturbed  judgment. 
In  the  present  case  this  difficulty  is  easily  avoided.  AA^e 
take  our  A^ermilion  disk  to  a room  illuminated  with  ordinary 
daylight,  and  reduce  its  luminosity  by  combining  it  with  a 
black  disk,  as  indicated  in  Fig.  91.  AA^hen  the  compound 
black  and  red  disk  is  set  into  rapid  rotation,  we  in  effect 
spread  out  over  the  Avhole  surface  of  the  disk  the  small 
amount  of  red  light  Avhich  is  reflected  from  the  exposed  red 
sector  ; its  luminosity  can  thus  be  reduced  to  any  desirable 
extent.  It  Avill  be  found,  when  we  combine  10  parts  of 


* See  Chapter  III. 


MODES  OF  ARRANGING  COLOURS  IN  SYSTEMS.  211 


vermilion  with  90  of  black  in  this  way,  that  the  red  colour 
is  converted  into  a chocolate-brown  that  bears  no  close  re- 
semblance to  the  original  hue.  It  can  be  objected  to  this 
mode  of  darkening  colours  that  there  may  be  in  the  black 
disk  something  that  exercises  a peculiar  effect  on  the  result. 
If,  however,  we  analyze  the  faint  light  which  comes  from 


Fig.  91.— Disk  with  10  Parts  Vermilion  and  90  Parts  Black:  giyes  by  rotation  a dark- 
reddish  chocolate-brown. 

the  black  disk  with  the  aid  of  a prism,  we  shall  find  that  it 
is  essentially  white,  all  the  prismatic  colours  being  present. 
Or  we  may  expose  our  black  disk  to  sunlight,  analyze  its 
light  with  the  prism,  and  compare  this  analysis  with  that 
obtained  from  a piece  of  white  paper  not  exposed  to  sun- 
light and  shaded  from  the  diffuse  daylight.  When  by  this 
proceeding  the  luminosities  of  the  black  and  white  papers 
have  been  equalized,  it  will  be  found  that  the  colours  which 
they  furnish  to  the  prism  differ  very  little.  The  fact  that 
the  black  disk  darkens  the  red  one  as  it  would  be  darkened 
by  a dark  room  can  also  be  proved  in  another  way  : If  we 
first  darken  a room  just  as  much  as  we  please,  and  then 
contrive  an  aperture  opening  into  it  a little  larger  than  our 
red  disk,  and  arrange  matters  so  that  not  much  light  enters 
the  room  through  this  aperture,  then  we  can  virtually  mix 
the  darkness  of  this  room  with  the  red  light  of  our  disk. 
We  simply  place  a red  sector  on  the  rotation  machine  ar- 
ranged in  front  of  the  opening  into  the  dark  room,  as  indi- 
cated in  Fig.  92,  aud  set  the  coloured  sector  into  rapid 


212 


MODERN  CHROMATICS. 


rotation.  We  now  have  the  red  light  of  a small  portion  of 
the  red  disk  spread  out  over  the  darkness  due  to  the  dark- 
ened room,  and  the  result  is  the  same  ; we  have  the  same 
chocolate-brown  (Fig.  93).  The  use  of  the  black  disk  being 
thus  justified,  we  may  employ  it  further  in  our  investiga- 
tion ; and  we  shall  find  that  by  simply  reducing  the  red 

lio;ht  of  our  vermilion  disk 
with  its  aid,  we  produce  not 
only  a series  of  dark,  dull 
reds,  but  a number  of  rich, 
peculiar-looking  b r o w n s . 
When  we  make  a corre- 
sponding set  of  experiments 


Fio  92.— Red  Sector  tirronped  with  Dark 
Room  as  a Rackground. 


Fig.  93.— Shows  the  appear- 
ance presented  when  the 
sector  of  Flir.  92  is  made  to 
rotate  rapidly. 


with  our  red-lead  disk,  we  obtain  a series  ol  reddish,  n aim- 
looking  browns;  the  chrome-yellow  f " y* 

stratic'e-looking,  dull  yellows  and  dark  olno-gree  - , 
cn-een  and  blue  disks  furnish  sets  of  dark  ^een  and  blue 
Tones.  Experiments  like  these  show  what  changes  are  pro- 
ved simjly  by  reducing  the  intensity  of  the  coloured 
licht  without  in  anv  other  way  tampering  with  it. 

' Br  a corresponding  set  of  experiments  we 
effects  of  mixins  white  light  with  our  coloured  light  . we 
need  only  combine  the  coloured  disk  with  one  that  is  w ^ e 
and  rapid  rotation  gives  us  the  desired  result.  In  this  way 


MODES  OF  AERANGING  COLOURS  IN  SYSTEMS.  218 


we  can  produce  a series  of  pale  tints  that  are  reddish,  green' 
ish,  or  bluish,  according  to  the  disk  employed. 

Thus,  either  by  reducing  the  luminosity  of  our  coloured 
light  or  by  mixing  it  with  more  or  less  white,  a great  num- 
ber of  tints  can  be  produced  ; but  we  soon  find  that  to 
match  many  natural  colours  it  is  necessary  to  employ  both 
these  proceedings  simultaneously.  By  combining  with  our 
coloured  disk  a black  and  a white  disk,  we  then  become 
able  to  imitate  a far  greater  number  of  the  pale,  indescriba- 
ble tints  of  natural  objects.  To  make  our  power  more 
complete,  we  ought  also  to  be  able  at  will  to  alter  the  wave- 
length  of  the  light  reflected  from  the  coloured  disk.*  There 
are,  however,  practical  difiiculties  which  prevent  us  from 
making  these  changes  in  a definite  and  perfect  manner,  and 
we  find  ourselves  finally  driven  to  abandon  our  very  conve- 
nient and  instructive  disks,  and  to  turn  for  help  to  the  col- 
ours of  the  solar  spectrum.  These  colours  are  pure  in  the 
sense  of  being  free  from  any  admixture  of  white  light,  their 
luminosity  can  be  varied  to  any  extent,  and  the  lengths  of 
the  various  waves  which  generate  them  have  been  measured 
with  a high  degree  of  accuracy  ; we  also  can  mix  white 
light  with  them  at  our  pleasure.  With  the  colours  of  the 
spectrum,  and  a purple  formed  by  mixing  the  red  and  violet 
of  the  spectrum,  we  can  match  any  colour  whatsoever,  pro- 
vided we  are  allowed  to  increase  or  diminish  the  luminosity 
of  our  spectral  hues,  and  to  add  the  necessary  amount  of 
white  light.  This  fact  furnishes  us  with  a clue  toward  a 
classification  of  colours.  The  series  red,  orange,  yellow, 
green,  blue,  violet,  and  purple  is  one  which  returns  on  it- 
self, and  hence  can  be  arranged  in  the  form  of  a circle,  as 
was  first  done  by  Newton.  In  making  a colour-chart  we 
can  place  the  complementary  colours  opposite  each  other, 
and  white  in  the  middle  ; the  pure  colours  of  the  spectrum 
will  be  situated  on  the  circumference  of  the  circle,  and  the 

* For  an  account  of  the  effects  produced  by  alteration  of  wave-length, 
see  Chapter  III. 


214 


MODERN  CHROMATICS. 


mixtures  of  these  with  white  will  lie  nearer  the  centre,  as  is 
roughly  indicated  in  Fig.  94.  A chart  of  this  kind  will 
contain  all  colours  that  are  possible  under  a given  degree  of 
illumination,  arranged  in  an  orderly  manner.  In  the  sector 
devoted  to  the  reds  we  shall  find  along  the  circumference 
every  kind  of  pure  red,  from  purple-red  to  orange-red,  and, 
as  we  advance  inward  toward  the  centre  of  the  circle,  a 
great  number  of  tints  produced  by  mixing  these  various 
reds  with  increasing  quantities  of  white.  It  is  the  same 


Fig.  94— The  colours  of  the  spectrum  are  supposed  to  be  on  the  circumference  of  the 
circle,  and  the  mixtures  of  these  with  increasing  quantities  of  white  are  in  the  inte- 
rior. White  is  at  the  centre.  (The  spaces  for  most  of  the  pale  or  greyish  tints  are 
left  blank,  for  want  of  room.) 


wdth  all  the  other  pure  colours  : every  possible  hue  and 
tint  belonging  to  the  adopted  grade  of  illumination  will  be 
found  somewhere  within  the  circle  ; all  the  manifold  greens 
and  blues,  also  the  whole  range  of  purples,  from  purplish- 
red  to  purplish-violet — all  will  be  represented.  At  the 
start,  one  of  our  conditions  was  that  complementary  col- 
ours should  be  opposite  each  other  ; hence  we  must  give  to 
our  blue  not  only  the  right  hue,  but  a luminosity  such  that 
it  is  able  exactly  to  neutralize  the  yellow^  Avhich  is  opposite 
to  it.  These  two  colours  must  also  have  a luminosity  such 


MODES  OF  ARRANGING  COLOURS  IN  SYSTEMS.  215 


that,  wlien  they  are  mixed,  the  mixture  will  furnish  a white 
which  is  twice  as  bright  as  that  at  the  centre  of  the  circle. 
The  same  must  be  true  of  all  the  other  23airs  of  complemen- 
tary colours,  whether  situated  on  the  circumference  or  in 
the  interior  of  the  circle.  This  mode  of  operating  gives  us 
a chart  in  which  all  the  colours  of  corresponding  intensity 
are  well  arranged  with  regard  to  each  other,  and  it  enables 
us  at  a glance  to  see  some  of  the  relations  of  the  colours  to 
each  other.* 

In  the  construction  of  this  colour-chart  we  imagined  the 
brilliant  colours  of  the  spectrum  to  be  situated  on  the  cir- 
cumference of  the  circle,  and  as  we  advanced  toward  the 
centre  a larger  and  larger  proportion  of  white  light  was  to 
be  mixed  with  them.  Suppose  now  we  diminish  somewhat 
the  luminosity  of  our  spectral  colours  ; this  will  change 
every  tint  in  the  chart  correspondingly,  and  also  the  central 
white  ; all  will  be  darkened  proportionately  ; we  shall  ob- 
tain a new  colour-chart,  having  less  luminosity,  but  in  other 
respects  closely  resembling  the  first.  With  a further  re- 
duction of  the  light  a corresponding  result  will  again  be 
reached  ; and,  as  we  continue  the  process,  we  go  on  accumu- 
lating new  colour-charts,  each  being  darker  than  the  last. 
Our  two  limits  evidently  are  the  brilliant  colours  of  the 
spectrum  on  the  one  hand  and  total  blackness  on  the  other  ; 
between  these  we  shall  be  able  to  place  a series  of  several 
hundred  colour-charts,  differing  perceptibly  in  luminosity, 
but  in  other  respects  resembling  the  original  type  as  nearly 
as  may  be. 

If  we  arrange  this  whole  series  of  charts  one  above  the 
other  in  proper  order,  the  most  luminous  one,  or  that  with 
which  we  started,  being  placed  at  the  top,  we  shall  obtain  a 
cylinder  which  will  contain  within  itself  an  immense  series 
of  colours.  The  axis  of  the  cylinder  at  the  top  will  be 

* In  this  case  we  consider  colours  equally  intense  which,  when  mixed, 
neutralize  each  other  and  produce  the  same  white.  The  luminosity  of  such 
colours,  measured  by  the  eye,  will  often  be  quite  difleront. 

10 


216 


MODERN  CHROMATICS. 


white,  and  as  we  descend  it  will  pass  through  a great  series 
of  darkening  greys,  finally  to  end  in  black.  If  we  make  a 
vertical  section  of  the  cylinder,  its  appearance  then  will  be 
of  the  nature  roughly  indicated  in  Fig.  95,  the  axis  darken- 
ing as  we  descend,  also  the  charts.  Now,  we  know  it  to  be 
a fact  that,  as  coloured  surfaces  are  more  and  more  feebly 
illuminated,  so  does  the  number  of  tints  which  we  can  dis- 


Fiq.  95. — Section  of  a Pile  of  Colour- 
Charts,  the  most  luminous  ones  be- 
ing at  the  top. 


Fig.  96. — Section  of  a Colour-Cone. 


tinguish  on  them  constantly  decrease.  Hence,  in  the  case 
of  our  cylindrical  pile  of  charts,  smaller  surfaces  will  an- 
swer equally  well  for  the  display  of  the  darker  tints,  and 
we  may  as  well  reduce  our  cylinder  to  a cone,  as  indicated 
in  Fig.  96.  This  colour-cone  is  analogous  to  the  colour- 
pyramid  which  was  described  by  Lambert  in  1772. 

It  will  be  remembered  that  we  began  our  colour-chart 
with  the  colours  of  the  spectrum,  and  these  colours,  along 
with  their  mixtures  with  white,  constituted  the  base  of  our 
cone.  The  colours  of  the  spectrum  in  this  case  were  sup- 
posed to  have  only  a moderate  degree  of  brightness,  such 
as  would  be  suitable  for  prolonged  observation.  These  are 
the  brightest  colours  thus  far  contained  in  our  cone,  but 


MODES  OF  ARRANGING  COLOURS  IN  SYSTEMS.  217 


they  are  by  no  means  the  brightest  colours  that  we  are  able 
to  see  ; above  them  occur  a great  series  of  hues  which  we 
can  more  or  less  perfectly  distinguish.  As  our  ability  to 
discriminate  very  luminous  colours  diminishes  as  their  lumi- 
nosity increases,  the  colour-charts  devoted  to  this  new  set 
may  be  treated  like  those  used  for  the  darker  colours.  In 
this  way  we  once  more  obtain  a second  cone,  which  will  be 
placed  over  the  one  before  described.  At  the  apex  of  this 
last  cone  we  have  the  brightest  white  which  the  eye  is 
capable  of  perceiving  ; a little  below  and  around  it  are  sit- 
uated a series  of  very  luminous  spectral  colours  and  a pur- 
ple, all  being  so  bright  as  to  differ,  so  far  as  the  eye  is 
concerned,  not  much  from  a brilliant  white  ; farther  down 
on  the  sides  of  the  cone  are  still  quite  bright  colours,  and 
in  its  interior  their  mixtures  with  white.  In  this  double 
cone,  then,  we  are  at  last  able  to  include  all  the  colours 
which  under  any  circumstances  we  are  able  to  perceive ; 
they  are  arranged  in  an  orderly  manner,  which  at  once  ex- 
hibits their  hues  and  luminosities,  and  the  amount  of  white 
light  that  has  been  mixed  with  them.  They  are  arranged 
throughout  in  complementary  pairs,  and  some  of  their  other 
relations  to  each  other  are  plainly  exhibited. 

INow,  a word  with  regard  to  the  possibility  of  executing 
this  colour-cylinder  or  the  double  cone.  In  the  first  place, 
we  have  no  pigments  with  which  we  can  at  all  properly  rep- 
resent the  colours  of  the  spectrum  even  when  their  luminos- 
ity is  quite  moderate  ; our  best  pigments  all  reflect  more  or 
less  white  light  mixed  with  their  coloured  light.  If  with  their 
aid  we  undertook  to  construct  a colour-chart,  we  should  not 
only  be  obliged  to  descend  in  the  cone,  Fig.  96,  a good  dis- 
tance toward  its  black  apex,  but  besides  this  our  chart 
would  be  smaller  than  the  section  of  the  cone  at  that  point, 
owing  to  the  presence  of  the  foreign  white  light  reflected 
by  the  pigments.  It  would  be  next  to  impossible  to  pre- 
pare pigments  of  different  colours  suitable  even  for  the  pro- 
duction of  a single  chart  of  the  series  ; for  it  would  be  ne- 


218 


MODERN  CHROMATICS. 


cessary  that  they  should  be  right  in  the  matter  of  hue, 
luminosity,  and  greater  or  less  freedom  from  white  light. 

There  are  still  other  objections  to  the  system  as  just 
proposed.  It  furnishes  us  with  no  means  of  giving  the  col- 
ours a proper  or  rational  distribution  on  the  circumference 
of  the  circle  ; we  do  not  know  whether  the  yellow  is  to  be 
placed  90°  from  the  red  or  at  some  other  distance  ; the 
same  is  true  with  regard  to  the  angular  distribution  of  all 
the  other  colours  ; the  system  gives  us  no  information  on 
this  point.  It  also  gives  us  no  information  about  the  effects 
that  are  produced  by  mixing  together  colours  that  are  not 
complementary. 

There  is  another  mode  of  attacking  this  problem  which 
has  been  much  used  of  late,  and  which  offers  certain  advan- 
tages to  the  student  of  colour.  Let  us  suppose  that  we 
place  red  of  a certain  luminosity  at  R,  and  green  of  the 
same  luminosity  at  G,  Fig.  97  ; then  along  the  line  R G 
we  can  arrange,  or  imagine  to  be  arranged,  all  possible 
mixtures  of  these  two  colours.  To  do  this  we  imagine  that 
R and  G have  certain  weights  corresponding  to  their  lumi- 
nosities (or  to  the  quantities  of  them  which  in  a particular 
case  we  employ),  and  then  proceed  as  if  we  had  before  us 

R G 


FiCr.  07.— Alonsr  the  line  E G we  can  arrange 
all  the  mixtures  of  red  and  preen.  The 
diapram  represents  the  case  where  equal 
parts  of  red  and  preen  are  employed. 

Then  the  point  of  support  (mixture-point) 
must  be  in  the  centre  of  the  line  E G. 

a mechanical  problem.  An  example  will  make  this  plain  : 
Let  the  luminosity  of  the  red  and  green  both  be  10  ; we 
now  mix  5 parts  of  red  with  5 of  green,  and  obtain  a yel- 
low ; the  position  of  this  yellow  will  be  at  L , Fig.  98,  half 


R V G 

* 1 • 

Fig.  os  — A mixture  of  equal  parts 
of  red  and  preen  furnishes  a yel- 
low; the  position  of  this  yellow  on 
the  line  of  mixtures,  E G,  is  at  Y. 


MODES  OF  ARRANGING  COLOURS  IN  SYSTEMS.  219 


way  between  R and  G.  We  put  the  position  of  yellow  at 
Y because,  in  order  that  5 parts  of  red  may  balance  5 parts 
of  green,  the  point  of  support  must  be  half  way  between  R 
and  G.  We  have  now  determined  accurately  on  the  line 
R G the  position  of  a yellow  made  by  mixing  equal  parts 
of  our  original  red  and  green.  This  yellow,  made  by 
mixing  5 parts  of  red  and  5 pf  green,  will  also  have  the 
same  luminosity  as  the  original  red  or  green.  If  we  mix  7 
parts  of  red  with  3 of  green,  the  position  of  the  mixture 
will  be  at  O,  Fig.  99.  If  we  divide  up  the  line  R G into 


FfG.  99.— Seven  parts  of  red  are  mixed  with  three  parts  of  green  ; the  mixture  is  orange 
in  colour,  and  situated  on  the  line  K G at  O. 

10  equal  parts,  then  O will  be  distant  from  R by  3 parts, 
but  from  G by  7 parts  ; for  in  this  way  alone  a balance  can 
be  obtained.  In  general  we  shall  always  obtain  a balance 
or  equilibrium  when  the  weight  of  R multiplied  by  its  dis- 
tance from  O is  equal  to  the  weight  of  G multiplied  by  the 
distance  of  G from  O.  In  this  last  case  (Fig.  99)  the  mix- 
ture at  O will  be  orange  in  colour,  and  will  have  a lumi- 
nosity identical  with  the  original  red  and  green.  So  we 
can  go  on  filling  up  the  line  R G with  all  possible  mixtures 
of  the  original  red  and  green.  Flow,  this  is  a process 
which  can  actually  be  carried  out  in  practice.  We  can 
take  for  our  red  a vermilion  disk,  and  then  select  for  our 
green  a disk  having  a colour  as  nearly  as  possible  of  the 
same  luminosity,  and  by  the  method  of  rotation  produce  all 
the  tints  above  indicated.  We  can  copy  these  tints,  and 
arrange  the  copies  along  the  line  R G ; or,  if  we  do  not 
wish  to  take  this  trouble,  we  can  at  least  always  reproduce 
at  will,  with  aid  of  the  red  and  green  disks,  any  of  the 
tints  belonging  on  the  line  R G.  This  explanation  Avill 
serve  to  render  clear  the  fundamental  idea  on  which  the 
colour-diagram  of  Newton  and  Maxwell  rests. 


220 


MODERN  CHROMATICS. 


Thus  far  we  have  employed  only  two  colours,  red  and 
green,  and  have  been  able  by  mixing  them  in  different  pro- 
portions to  obtain  various  hues  of  orange,  yellow,  and 
greenish-yellow.  If  we  wish  to  include  the  other  colours, 
blue,  violet,  purple,  and  the  mixtures  of  all  the  colours 
with  white,  we  must  employ  at  the  start  three  instead  of 
two  colours.  Maxwell  selected  vermilion,  emerald-green, 
and  ultramarine-blue,  since  according  to  his  researches  they 
approximately  represent  the  three  fundamental  colours. 
These  he  placed  at  the  three  angles  of  an  ecpiilateral  trian- 
gle, and  ascertained  in  a manner  afterward  to  be  explained 
the  position  of  white  (or  grey)  in  the  interior  of  the  trian- 
gle. Every  colour  that  can  be  obtained  by  mixing  red 
with  green  will  lie  on  the  line  joining  red  and  green  ; it  is 
the  same  with  green  and  blue,  also  with  red  and  blue.  Fig. 
100  shows  these  colours  disposed  along  the  sides  of  the  tri- 
angle ; they  are  also  so  arranged  that  complementary  col- 
ours are  opposite  each  other  ; white  is  in  the  interior,  and 
along  the  lines  joining  the  sides  with  the  centre  are  placed 
the  various  colours  mixed  with  more  and  more  white  as 
they  are  situated  nearer  to  tlie  centre.  The  colours  made 
by  mixing  red  with  green,  or  green  with  blue,  being  situ- 
ated on  the  sides  of  the  triangle,  are  consequently  as  a gen- 
eral tiling  nearer  to  the  position  of  white  than  the  three 
fundamental  colours  at  the  three  angles  of  the  triangle. 
This  indicates  in  a geometrical  manner  tlie  fact  that  the 
tints  produced  by  the  mixture  of  two  fundamental  colours 
are  paler,  or  mixed  with  more  white,  than  the  fundamentals 
themselves.  In  general,  in  a chart  of  this  kind,  the  farther 
we  go  from  the  centre  or  from  the  position  of  white,  the 
more  do  we  obtain  colours  which  are  free  from  admixture 
with  white.  The  angular  position  of  the  colours  is  to  some 
extent  arbitrary,  being  determined  partly  by  the  process  of 
mixture,  and  partly  by  the  assumption  that  particular  hues 
of  red,  green,  and  blue  represent  the  fundamental  colours. 
If  we  should  assume  as  our  fundamental  colours  red  lead. 


MODES  OF  ARRANGING  COLOURS  IN  SYSTEMS.  221 


grass-green,  and  violet,  the  angular  position  of  all  the  other 
colours  would  be  disturbed.  Still,  in  spite  of  this  and  other 
drawbacks,  the  colour-diagram  is  valuable  for  purposes  of 


BLUE 


Fig.  100. — Shows  the  colours  that  are  produced  by  mixing  red  with  green,  etc.  The 
pale  colours,  or  those  mixed  with  white,  are  situated  in  the  interior  of  triangle,  and 
grow  paler  as  they  are  nearer  W,  or  white. 

study,  and  enables  us  to  express  our  ideas  about  colour  in  a 
geometrical  form  and  with  a certain  degree  of  precision. 
It  is  easy,  for  example,  with  its  aid  to  ascertain  the  result 
of  mixing  together  any  of  the  colours  contained,  or  sup- 
posed to  be  contained,  in  it ; if  we  mix  equal  parts  of  red 
with  cyan-blue,  it  is  evident  by  inspection  that  the  result 
must  be  a whitish-purple  ; so  again  equal  parts  of  yellow 
and  cyan-blue  will  furnish  a whitish-green.  (See  Fig.  100.) 
With  the  aid  of  this  diagram  we  can  accomplish  even 
more  : we  can  mix  together  any  number  of  colours,  and 


222 


MODERN  CHROMATICS. 


ascertain  the  position  and  consequently  the  tint  of  the 
resultant  hue.  We  select  any  two,  join  them  by  a line, 
and  ascertain,  as  previously  explained,  the  position  of  the 
mixture  tint ; we  then  join  the  third  colour  by  a straight 
line  with  the  point  just  ascertained,  and  again  construct 
the  position  of  the  second  mixture,  and  so  on.  An  account 
of  the  mode  of  making  a colour-diagram  of  this  kind  will 
be  found  in  the  appendix  to  this  chapter,  where  the  method 
will  also  be  explained  which  Maxwell  employed  for  the 
introduction  of  colours  more  or  less  luminous  than  the  three 
fundamental  ones. 

The  colour-charts  which  thus  far  have  actually  been 
published  and  laid  before  the  world  have  been  of  a different 
character  from  those  above  indicated,  and  are  calculated  to 
display  the  effects,  not  of  mixing  coloured  light,  but  col- 
oured pigments.  Among  the  older  attempts  we  may  men- 
tion those  of  Le  Blond  in  1735,  and  of  Du  Fay  in  1737.  In 
1758  T.  Mayer  published  an  account  of  his  experiments. 
As  three  fundamental  colours  he  selected  vermilion,  a bright 
yellow,  and  smalt-blue.  Lambert  in  1772  used  for  the  con- 
struction of  his  pyramid  carmine,  gamboge,  and  Prussian- 
blue.  These  colour-charts  were  constructed  by  mingling 
weighed  portions  of  the  fundamental  pigments  and  of  lamp- 
black in  such  a manner  as  to  obtain  as  great  a variety  of 
tints  as  possible,  which  were  then  arranged  in  an  orderly 
series.  The  very  beautiful  colour-charts  of  Chevreul  are 
essentially  of  the  same  nature  with  those  just  mentioned. 
Chevreul  employed  a circle  with  three  radii  which  were 
120°  apart,  and  placed  on  these  radii  red,  yellow,  and  blue  ; 
the  hues  of  these  colours  were  copied  from  certain  portions 
of  the  prismatic  spectrum  which  were  selected  as  standards. 
Between  red  and  yellow  the  various  hues  of  orange  and 
orange-yellow  were  introduced  ; between  yellow  and  blue 
the  greens,  the  purples  being  situated  between  violet  and 
red.  This  constitutes  the  first  chromatic  circle,  which  con- 
tains also  the  purest  and  most  intense  colours.  In  the  sec- 


MODES  OF  ARRANGING  COLOURS  IN  SYSTEMS.  223 


ond  circle  the  same  colours  are  shown  mixed  with  a small, 
definite  amount  of  black  ; the  third  circle  is  like  the  sec- 
ond, only  still  more  darkened,  and  so  on.  There  are  ten 
of  these  circles,  each  containing  seventy-two  tints  ; com- 
plementary colours  are  situated  opposite  each  other.  Be- 
sides the  circles,  there  are  charts  containing  colours  arranged 
in  parallel  hands  ; these  are  intended  to  exhibit  the  effects 
of  mixing  black  and  white  with  the  colours  contained  in 
the  first  circle.  They  consist  of  twenty-two  bands,  the  0 
band  being  white,  the  21st  black,  and  the  10th  band  the 
same  as  the  corresponding  colour  in  the  first  circle.  Start- 
ing, for  example,  from  the  10th  band,  as  we  move  to  0 the 
colour  grows  continually  paler,  being  mixed  with  more  and 
more  white  ; if  we  advance  in  the  other  direction,  the  col- 
our becomes  darker,  and  ends  finally  in  black.  There  are 
seventy-two  sets  of  these  bands,  also  one  for  black  and 
white. 

The  ideas  upon  which  this  ctart  is  based  are  not  only 
in  the  main  arbitrary,  but  also  vague,  and  the  execution  of 
the  sample  examined  by  the  author  left  much  to  be  desired. 
We  can  not  regard  this  colour-chart  as  a true  step  toward 
a philosophical  classification  of  colours,  but  rather  as  a 
more  elaborate  repetition  of  the  work  of  Mayer,  Lambert, 
and  Bunge.  In  point  of  fact,  our  knowledge  of  colour 
and  our  means  of  experimenting  on  it  are  not  at  present 
sufficiently  advanced  to  enable  us  even  to  propose  a plan 
for  a truly  philosophical  classification,  and  between  the  pro- 
posal and  its  execution  there  would  be  many  weary  steps. 
Hence,  the  matter  contained  in  this  chapter  and  its  appen- 
dix is  rather  to  be  regarded  as  setting  forth  the  problem 
than  as  attempting  its  solution. 


224 


MODERN  CHROMATICS. 


APPENDIX  TO  CHAPTER  XIY. 

OOLOUE-DIAGEAMS. 

Following  a suggestion  of  Newton’s,  Maxwell  constructed  a 
diagram  in  which  the  colours  of  pigments  and  many  natural  objects 
can  be  laid  down  in  accordance  with  certain  principles  and  assump- 
tions presently  to  be  explained.  Now,  although  some  of  the  as- 
sumptions are  arbitrary,  yet  if  they  are  accepted  a chart  is  obtained 
which  presents  many  valuable  features  for  the  student  of  colour. 
The  nature  and  scope  of  this  colour-diagram  will  perhaps  best  be 
made  evident  by  tracing  the  actual  construction  of  one  as  made  by 
the  present  writer. 

Following  Maxwell,  vermilion,  emerald-green,  and  artificial 
ultramarine-blue  were  assumed  as  the  three  fundamental  colours, 
and  positions  at  the  three  angles  of  an  equilateral  triangle  assigned 
to  them.  The  length  of  each  side  of  the  triangle  was  200  divisions 
of  the  scale  employed.  The  first  step  was  to  determine  the  posi- 
tion of  white  in  the  triangle.  For  this  purpose  disks  of  vermilion, 
emerald-green,  and  ultramarine  w'ere  combined  as  shown  in  Fig. 
101,  smaller  central  disks  of  black  and  white  being  placed  on  the 


Fig.  101.— Compound  Disk  of  Vermilion,  Emerald-green,  Ultramarine,  White  and 
Black,  arranged  for  the  production  of  Grey. 


same  axis.  It  was  found,  when  the  colours  were  mixed  by  rapid 
rotation,  that  36*46  parts  of  vermilion,  with  33*76  of  emerald-green 
and  29*76  of  ultramarine,  gave  a grey  similar  to  that  obtained  by 
mixing  28*45  parts  of  white  with  71*55  of  black.  In  the  experi- 
ment 24*5  parts  of  white  were  actually  obtained ; but  this  was  cor- 


APPENDIX  TO  CHAPTER  XIV. 


225 


rected  by  adding  to  it  the  white  due  to  the  black  disk,  it  having 
been  previously  ascertained  that,  if  the  luminosity  of  the  white 
paper  composing  the  white  disk  was  taken  as  100,  that  of  the  black 
disk  was  6'24.  The  same  correction  was  made  in  all  the  cases  that 
follow.  The  equation  then  reads : 

36*46  R + SS-Ye  G + 29*76  B = 28*45  W (1). 

The  next  step  is  to  divide  up  the  line  R G,  Fig.  103,  in  the  ratio  of 
36*46  to  33*76: 

(36*46  + 33*76)  : 36*46  ::  200  ; 103*5. 

That  is,  if  we  mix  vermilion  and  emerald-green  in  the  propor- 
tion of  36*46  to  33*76,  the  mixture-point  {a)  lies  on  the  line  RG, 
and  is  103*5  divisions  distant  from  G,  Fig.  103.  This  point  is  the 
position  of  the  complement  of  the  fundamental  ultramarine,  B. 
We  now  connect  this  point  {a)  with  B by  a straight  line,  find  the 
length  of  the  line  to  be  173*5  divisions,  and  seek  the  mixture-point 
of  36*46  R + 33*76  G and  29*76  B,  which  is  obtained  by  the  follow- 
ing proportion : 

[(36*46  -1-  33*76)  -f  29*76]  *.  29*76  ::  173*5  : 51*64. 

Tliis  mixture-point  is  the  position  of  white ; for  vermilion,  em- 
erald-green, and  ultramarine,  when  mixed  in  the  above  proportions, 
produce  white.  Hence,  white  (W,  Fig.  103)  will  be  on  the  line 
<zB,  51*64  divisions  from  «.  (It  evidently  must  be  somewhere  on 
this  line,  for  at  the  two  extremities  of  the  line  are  colours  which 
are  complementary,  and  there  must  be  a mixture-point  on  the  line 
which  is  white.)  It  will  be  noticed  that  in  this  proceeding  the 
colours  have  been  heated,  according  to  N"ewton’s  suggestion,  as 
though  they  were  weights  acting  on  the  ends  of  lever-arms,  and 
these  arms  have  been  taken  of  such  lengths  as  to  bring  the  system 
into  equilibrium.  It  will  also  be  observed  that  it  has  been  assumed 
that  the  pigments,  vermilion,  emerald-green,  and  ultramarine,  have 
the  same  intensity,  or  that  equal  areas  of  them  have  the  same 
weight.  Thus,  36*46  ]>arts  of  vermilion  and  33*76  of  emerald-green, 
acting  on  a lever- arm  51*64  divisions  in  length,  balance  29*76  parts 
of  .ultramarine  acting  on  an  arm  with  a length  of  121*86  divisions. 
The  lever-arms  of  the  vermilion  and  emerald-green  passing  through 
W are  also  similarly  balanced,  and  the  whole  system  is  in  equilib- 
rium. 

The  white  or  grey  which  was  obtained  in  equation  (1)  was  the 


226 


MODERN  CHROMATICS. 


equivalent  of  100  parts  of  colour ; by  multiplying  28‘45  by  3-51  we 
obtain  100,  and  we  set  these  100  grey  units  in  the  place  of  28-45  W 
in  equation  (1),  and  obtain  what  Maxwell  calls  the  corrected  value 
of  the  white.  The  factor  3-51  is  called  the  coefficient  of  the  white, 
and  is  used  to  establish  a relation  between  equation  (1)  and  those 
that  follow.  The  coefficients  of  vermilion,  emerald-green,  and 
ultramarine  have  at  the  outset  been  assumed  as  1,  and  hence  in  its 
corrected  form  equation  (1)  reads  thus : 

36-46  R + 33-'76  G -I-  29-76  B = 100  w (2). 

Wo  have  now  laid  down  upon  our  colour-diagram  the  position 
of  our  three  fundamental  colours  and  that  of  white,  and  are  pre- 
pared to  assign  positions  to  all  other  pigments  or  mixtures  of  pig- 
ments. For  example,  to  determine  the  position  of  pale  chrome- 
yellow,  a disk  covered  with  this  pigment  is  to  be  combined  with 
disks  of  emerald-green  and  ultramarine,  and  set  in  rotation  (Fig. 


Fio.  102.- 


Compound  Disk  of  nimmo-yellow,  Emerald-prcpn,  Tltramariiie, 
Black,  arranged  so  ns  to  give  a pure  Grey  by  rotation. 


White,  and 


102).  This  experiment  was  made,  and  the  following  equation  ob- 
tained : 

26-9  Y -f  12-6  G -f  60-6  B = 32-4  W -f-  67-6  b (3). 

Before  using  equation  (3),  it  is  necessary  to  bring  it  into  relation 
with  equation  (2),  and  the  first  step  is  to  express  the  value  of  the 
white  in  the  same  manner  as  in  equation  (2),  viz. : we  multiply  it 
by  the  coefficient  3-51,  and  obtain  in  this  way  the  value  of  the  cor- 
rected white,  or  113-87.  This  quantity  we  substitute  in  equation 
(3),  which  then  reads : 

26-9  Y + 12-5  G + 60-6  B = 113-87  w 


(4). 


APPENDIX  TO  CHAPTER  XIV. 


227 


We  must  now  introduce  a corrected  value  for  the  chrome-yellow,  so 
arranged  that  we  shall  have  the  same  number  of  units,  grey  or  col- 
oured, on  both  sides  of  the  sign  of  equality ; 

113-87  - (60-6  + 12-5)  = 40-77. 

40-77  is  theu  the  corrected  value  of  the  chrome-yellow,  and  equa- 
tion (3)  in  its  corrected  form  finally  reads : 

40-77  Y + 12-5  G -f  60*6  B = 113-87  w (5). 

To  obtain  the  coefficient  of  the  chrome-yellow,  we  divide  the  cor- 
rected value  by  the  original  value  : 

40-77 

= P51  = coef.  of  ch.-yel. 

26-9  . 

We  are  now  prepared  to  determine  the  position  of  chrome-yel- 
low in  the  diagram.  We  divide  the  line  B G into  two  parts  having 
the  ratio  of  12-5  to  60-6  : 

(12-5  -f  60-6)  : 12-5::  200  : 34-2. 

The  position,  then,  of  the  complement  of  chrome-yellow  is  on  the 
line  B G,  Fig.  103,  at  the  spot  marked  cobalt,  and  is  distant  from  B 
by  34*2  divisions ; the  distance  of  this  point  from  W is  found  by 
measurement  to  be  94-1  divisions.  We  connect  the  point  by  a 
straight  line  with  W,  and  produce  the  line  some  distance  beyond; 
the  position  of  chrome-yellow  will  be  on  this  line,  and  may  be 
found  by  the  following  proportion:  Weight  of  chrome-yellow: 
weight  of  emerald-green  and  ultramarine  ::  distance  of  emerald- 
green  and  ultramarine  : distance  of  chrome-yellow ; or, 

40-77  : (60-6  12-5)::  94-1  : 168'7. 

Chrome-yellow  is  consequently  distant  from  the  neutral  point  or 
white  168-7  divisions;  we  insert  it  in  the  diagram  along  with  its 
coefficient  1-51.  By  a corresponding  process  the  positions  and  coef- 
ficients of  a number  of  the  more  ordinary  colours  have  been  laid 
down  in  the  diagram.  See  Fig.  103.  If  the  diagram  is  examined, 
it  will  be  found  that  along  any  single  radius  the  pale  colours,  or 
those  mixed  with  much  white,  are  located  hearer  W than  those 
that  are  more  free  from  such  admixture;  it  will  also  be  noticed  that 
the  more  luminous  colours  have  higher  coefficients.  By  the  aid  of 
this  diagram  we  obtain  relative  measures  of  the  luminosity  and  sat- 
uration of  colours  on  the  same  or  on  closely  adjacent  radii ; the 


228 


MODERN  CHROMATICS. 


colours  also  have  angular  positions  assigned  to  them,  so  that  they 
are  fairly  defined  as  to  angular  position,  intensity,  and  greater  or 
less  freedom  from  white. 

It  is,  however,  to  be  remarked  that  the  construction  rests  upon 
several  more  or  less  arbitrary  assumptions,  as:  1.  That  vermilion, 
emerald-green,  and  ultramarine-blue  reaUy  correspond  to  the  three 
fundamental  colours.  If  we  substitute  in  place  of  them  other  col- 
ours, such  as  red  lead,  grass-green,  and  violet,  we  obtain  different 


APPENDIX  TO  CHAPTER  XIV. 


229 


angular  positions  for  all  tlie  colours  afterward  introduced,  and  also 
different  coefficients.  2.  The  assumption  that  vermilion,  emerald- 
green,  and  ultramarine  have  the  same  intensity  or  the  same  coeffi- 
cient is  quite  unwarranted,  the  intensity  of  emerald-green  being 
evidently  greater  than  than  of  ultramarine-blue.  From  this  it  fol- 
lows that  the  coefficients  and  distances  from  the  central  white,  W, 
are  not  comparable  along  different  radii. 

The  author  has  reconstructed  the  same  colour-diagram,  intro- 
ducing coefficients  which  represent  the  actual  luminosities  of  the 
three  fundamental  colours.  These  coefficients  were  obtained  by  the 
method  described  in  Chapter  III.  Vermilion,  emerald-green,  and 
artificial  ultramarine-blue  were,  as  before,  assumed  as  the  funda- 
mental colours,  and  placed  at  the  three  angles  of  an  equilateral  tri- 
angle. Taking  the  luminosity  of  white  paper  as  100,  the  luminosi- 
ties of  the  three  pigments  were  as  follows:  Vermilion,- 26*85 ; 
emerald- green,  48*58 ; ultramarine-blue,  7*57.  Introducing  these 
coefficients  into  equation  (1),  it  becomes : 

Vermilion.  Emerald-green.  Ultramarine.  White. 

(26*85  X *3646)  -I-  (48*58  x *3376)  d-  (7*57  x *2976)  = 28*44. 

That  is : 

9*8  verm,  -f  16*4  ern. -green  + 2*2  ult.  = 28*44  white (6). 

With  the  aid  of  equation  (6),  the  position  of  white  was  determined 
in  the  manner  previously  employed,  and  found  to  be  that  indicated 
in  Fig.  104,  being  only  13*55  divisions  distant  from  the  line  V G. 
Making  use  of  equation  (3),  the  coefficient  of  chrome-yellow  was 
determined  as  indicated  below,  X being  this  quantity : 

(48*58  X *125)  -f  (X  X *269)  -f  (7*57  x 0*606)=  32*4. 

X = 80*82. 

We  then  have — 

6*072  em.-green  -f-  21*74  ch.-yel.  + 4*587  ult.  = 32*4 (7). 

With  the  aid  of  this  last  equation  the  position  of  chrome-yellow  can 
be  laid  down  in  the  manner  previously  described.  In  Fig.  104  the 
positions  of  the  same  pigments  are  shown  which  were  employed  in 
tlie  first  colour- diagram.  Fig.  103 ; and  it  will  bo  noticed  that  white 
has  been  moved  toward  the  line  R G,  and  that  the  angular  positions 
of  the  colours  have  been  considerably  altered.  In  this  second  col- 
our-diagram the  coefficients  of  pigments  situated  along  different 


230 


MODERN  CHROMATICS. 


Fig.  104.— Maxwell’s  Diagram  as  reconstructed  by  the  Author,  correct  CoeflBcients  being 

employed. 


radii  are  comparable  with  each  other,  since  they  represent  the  lu- 
minosities of  the  pigments  compared  with  that  of  whitepaper  taken 
as  100. 

The  author  has  constructed  a new  kind  of  colour-diagram,  in 
which  the  colours  are  arranged  in  a different  manner  from  those 
just  described. 

Idea  of  the  Keic  Diagram. — Let  us  suppose  that  we  take  a cer- 
tain quantity  of  pure  red  light  and  locate  it  on  the  circumference  of 
a circle  at  R,  Fig.  105,  and  draw  the  diameter  RGB,  and  at  the 
point  G B locate  a quantity  of  pure  green-blue  light,  just  sufficient 
to  neutralize  the  red  light,  or  form  with  it  a mixture  which  appears 
to  the  eye  white.  The  position  of  white  will  then  be  at  the  centre 
of  the  circle,  or  at  TT.  The  red  and  green-blue  light  employed  will 
be  considered  equal  in  intensity,  though  in  actual  luminosities  they 


APPENDIX  TO  CHAPTER  XIV. 


231 


may  differ  considerably ; they  will,  in  point  of  fact,  relatively  to 
each  other,  have  equal  saturating  powers.  We  next  lay  down  on 
the  circumference  at  Y a certain  amount  of  pure  yellow  light,  draw 
the  diameter  Y B,  Eig.  105,  and  at  B locate  an  amount  of  blue  light 


just  sufficient  to  neutralize  it,  arranging  matters  so  that  the  yellow 
and  blue  light  when  mixed  shall  reproduce  a white  identical  in 
luminosity  with  that  furnished  by  the  mixture  of  the  red  and  green- 
blue.  The  yellow  and  blue  will  differ  greatly  in  luminosity,  but,  as 
they  neutralize  each  other,  will  be  considered  to  have  equal  inten- 
sity. Each  of  the  four  colours  will  also  be  considered  to  have  equal 
intensity  in  the  sense  in  which  the  word  has  just  been  employed; 
or,  instead  of  using  the  term  intensity,  we  may  say  that  each  of  the 
four  colours  will  have  corresponding  powers  of  saturation.  The 
same  will  be  true  of  any  other  colours  belonging  on  the  circumfer- 
ence. In  order  to  realize  this  idea,  and  to  obtain  means  of  assign- 
ing to  the  colours  proper  angular  positions,  some  other  considera- 
tions must  be  entertained.  Suppose  we  mix  the  yellow  located  at 

Y with  the  green-blue  located  at  G B : we  shall  by  varying  the  pro- 
portions finally  obtain  a mixture  which,  although  it  is  not  white, 
yet  will  be  paler  or  more  whitish  than  any  other  mixture;  this,  of 
course,  is  a well-known  fact.  In  the  practical  construction  of  the 
diagram,  it  is  assumed  that  this  most  neutral  mixture  will  bo  ob- 
tained when  the  whole  mass  of  the  yellow  at  Y is  mixed  with  the 
whole  mass  of  green-blue  at  G B;  and  it  is  evident  that,  even  if 
this  assumption  is  not  strictly  true,  it  will  approximate  to  the  truth 
just  in  proportion  as  the  angular  distance  between  R and  Y hap- 
pens to  be  a small  quantity.  If  the  angular  distance  between  R and 

Y is  a largo  quantity,  the  assumption  may  or  may  not  hold  good ; 


232 


MODERN  CHROMATICS. 


at  present  we  have  no  means  of  deciding  this  point.  We  will  take 
it  for  granted  till  the  contrary  is  proved,  and  from  the  most  neu- 
tral point,  we  draw  a perpendicular ; it  will  pass  through  the  centre 
of  the  circle,  or  through  the  position  of  white.  The  same  will  hold 
good  when  any  other  point  not  far  distant  from  R is  connected  by 
a straight  line  with  G B ; here  also  a perpendicular  drawn  from  the 
most  neutral  point  will  pass  through  white. 

Realization  of  the  Diagram. — In  order  to  construct  this  diagram 
it  is  necessary  to  prepare  three  coloured  disks  having  equal  intensi- 
ties, in  the  sense  above  employed,  or  equal  saturating  powers. 
These  disks  must  also  have  such  colours  that  by  optical  mixture 
they  may  be  capable  of  furnishing  white  light.  The  colours  select- 
ed were  red  lead,  a grass-green,  and  artificial  ultramarine-blue. 
The  green  disk  was  combined  with  the  blue  disk,  and,  by  a rather 
elaborate  series  of  experiments,  it  was  ascertained  that  the  most 
neutral  mixture  was  obtained  when  equal  areas  were  optically 
mixed,  from  which  it  was  concluded,  according  to  the  fundamental 
assumption,  that  the  saturating  powers  of  the  two  disks  were  equal. 
After  several  trials,  a similar  equality  was  established  between  the 
green  disk  and  one  painted  with  slightly  darkened  red  lead.  These 
disks  when  combined  gave  the  following  equation  : 

23-06  red  lead  -t-  42-16  green  -1-  34-76  blue  = 22-1  white. 

The  coefficients  of  the  three  colours  were  taken  as  unity,  since  the 
colours  had  e(pial  saturating  powers.  The  relative  areas  of  the  col- 
ours in  the  above  equation  were  then  used  as  weights,  and  furnished 
the  means  of  determining  the  positions  of  the  three  colours  on  the 
circumference  of  a circle  in  which  white  was  placed  at  the  centre. 
This  was  accomplished  by  placing  the  three  colours  at  such  angular 
distances  apart  as  brought  the  whole  system  into  equilibrium ; for 
example,  if  the  weights  had  been  equal,  the  angular  distance  of  the 
three  points  would  have  been  120°.  The  proper  angular  distances 
being  now  laid  down,  the  positions  of  darkened  red  lead,  grass- 
green,  and  ultramarine  were  determined;  and  with  their  aid  the 
positions  of  other  pigments  could  be  ascertained  by  the  process  of 
mixture  previously  explained.  (See  Fig.  106.)  The  points  laid  down 
in  this  diagram  indicate  colour  or  hue  by  angular  position,  and 
saturation  or  intensity  by  greater  or  less  distance  from  W.  The 
relative  amounts  of  white  light  reflected  by  the  pigments  situated 
on  any  particular  radius  can  easily  be  determined,  since  distance 


APPENDIX  TO  CHAPTER  XIV. 


233 


from  the  centre  measures  the  amount  of  coloured  light  reflected, 
and  the  total  amount  of  coloured  and  white  light  reflected  can  he 
measured  by  the  process  described  in  Chapter  III.  We  have,  how- 
ever, at  present  no  means  of  generalizing  this  process  and  applying 


Fia.  106.— Saturation-Diagram  according  to  Rood.  The  three  colours  used  in  its  con- 
struction are  marked  8. 


it  to  colours  situated  on  different  radii,  since  we  have  not  the  power 
of  ascertaining,  for  example,  whether  our  standard  yellow  disk  at  Y 
reflects  the  same  amount  of  white  light  with  the  standard  red  disk 
at  R,  or  more  or  less;  wo  know  that  they  reflect  corresponding 
quantities  of  coloured  light,  but  nothing  more.  Before  we  can 


234 


MODERN  CHROMATICS. 


solve  this  problem  it  will  be  necessary  for  us  to  know  the  relative 
luminosity  of  all  the  pure  colours  (free  from  white  light)  which,  ac- 
cording to  the  construction,  fall  on  tbe  circumference  of  the  circle, 
and  this  could  only  be  ascertained  by  an  especial  study  of  the  spec- 
tral colours  with  reference  to  this  point;  but  no  such  study  has  yet 
been  made.  We  know  that  corresponding  amounts  of  yellow  and 
greenish-yellow  have  not  only  higher  degrees  of  luminosity  than 
their  complements,  blue  and  violet,  but  even  higher  than  any  of  the 
other  colours;  but  thus  far  no  quantitative  determinations  have 
been  made.  Fig.  lOG  exhibits  a diagram  of  the  kind  just  described, 
containing  the  same  colours  or  pigments  pre\dously  employed : it  is 
perhaps  best  called  a saturation-diagram. 


CHAPTER  XV. 


CONTRAST. 

We  have  now  studied  with  some  care  the  changes  which 
coloured  surfaces  experience  when  viewed  under  various 
kinds  of  illumination,  or  when  modified  in  appearance  by 
the  admixture  of  more  or  less  white  or  coloured  light.  The 
appearance  which  a coloured  surface  presents  to  us  can, 
however,  be  altered  very  materially  by  a method  which  is 
quite  different  from  any  of  those  that  have  thus  far  been 
mentioned  : we  can  actually  change  colour  to  a considera- 
ble extent  without  at  all  meddling  with  it  directly,  it  being 
for  this  purpose  only  necessary  to  alter  the  colour  which 


GREEN 


RED 


Fig.  107. — Sheets  of  Eed  and  Green  Paper  with  Eed  Squares. 

lies  adjacent  to  it.  We  can  satisfy  ourselves  of  this  fact 
by  a very  simple  experiment.  If  we  cut  out  of  a sheet  of 
red  paper  two  square  pieces  an  inch  or  two  in  size,  and  then 
place  one  of  them  on  a sheet  of  red  and  the  other  on  a sheet  of 
green  paper,  as  indicated  in  Fig.  107,  it  will  be  found  that 
the  red  square  on  the  red  paper  will  not  appear  nearly  so 


236 


MODERN  CHROMATICS. 


brilliant  and  saturated  in  colour  as  that  placed  on  the  green 
ground,  so  that  the  observer  will  be  disposed  to  doubt 
whether  the  two  red  squares  are  really  identical  in  hue. 
By  a somewhat  analogous  proceeding  we  can  cause  a sur- 
face which  properly  has  no  colour  of  its  own,  which  is  real- 
ly grey,  to  appear  tinted  red,  blue,  green,  etc.  These 
changes  and  others  of  a like  character  are  produced  by 
what  is  called  contrast,  and  are  partly  due  to  actual  effects 
generated  in  the  eye  itself  and  partly  to  fluctuations  in  the 
judgment  of  the  observer.  The  subject  of  contrast  is  so 
important  that  it  will  be  worth  while  to  make  a somewhat 
careful  examination  of  the  laws  which  govern  it  ; and  for 


cured  Image. 


this  purpose  it  will  be  well  for  the  reader  to  repeat  some  of 
the  simple  experiments  described  below. 

If  we  place  a small  piece  of  bright-green  paper  on  a 
sheet  of  grey  drawing-paper,  in  the  manner  indicated  in 
Fig.  108,  and  then  for  several  seconds  attentively  look  at 
the  small  cross  in  the  centre  of  the  green  slip,  we  shall  find, 
on  suddenly  removing  it,  that  in  its  place  a faint  image  of 
a rose-red  colour  makes  its  appearance.  Fig.  109.  This  red 
image  presently  vanishes,  and  the  grey  paper  resumes  its 
natural  appearance.  The  rose-red  ghost  which  is  thus  de- 
veloped has  a colour  which  is  complementary  to  that  which 
called  it  into  existence,  and  this  will  also  be  the  case  if  we 


CONTRAST. 


237 


employ  little  squares  of  other  colours  : red  will  give  rise  to 
a greenish-blue  image,  blue  to  a yellow,  violet  to  a green- 
ish-yellow, etc.,  the  colour  of  the  image  being  always  com- 
plementary to  that  which  gave  rise  to  it.  On  this  account 
these  images  are  called  negative,  since,  as  far  as  colour 
goes,  they  are  just  the  reverse  of  the  images  which  are  first 
presented  to  the  eye  of  the  observer.  They  are  also  often 
spoken  of  in  older  treatises  on  optics  as  “ the  accidental  col- 
ours.” It  is  quite  easy  to  explain  their  production  with  the 
aid  of  the  theory  of  Young  and  Helmholtz.  Let  us  take  as 
an  example  the  experiment  just  described.  According  to 
our  theory,  the  green  light  from  the  little  square  of  paper, 
acting  on  the  eye,  fatigues  to  some  extent  the  green  nerves 
of  the  retina,  the  red  and  violet  nerves  meanwhile  not 
being  much  affected.  When  the  green  paper  is  suddenly 
jerked  away  by  the  string,  grey  light  is  presented  to  the 
fatigued  retina,  and  this  grey  light  may  be  considered  to 
consist,  as  far  as  we  are  concerned,  of  red,  green,  and  violet 
light.  The  red  and  violet  nerves,  not  being  fatigued,  re- 
spond powerfully  to  this  stimulus  ; the  green  nerves,  how- 
ever, answer  this  new  call  on  them  more  feebly,  and  in  con- 
sequence we  have  presented  to  us  mainly  a mixture  of  the 
sensations  red  and  violet,  giving  as  a final  result  rose-red  or 
purplish-red.  The  green  nerves,  of  course,  are  not  so  fa- 
tigued that  they  do  not  act  at  all  when  the  grey  light  is 
presented  to  them,  but  the  only  effect  that  their  partial 
action  has  is  to  render  the  rose-coloured  image  somewhat 
pale  or  whitish  in  appearance.  The  fatigue  of  the  optic 
nerve  mentioned  here  does  not  differ  essentially  from  that 
which  it  undergoes  constantly,  even  under  the  conditions  of 
ordinary  use,  where  the  waste  is  continually  made  good  by 
the  blood  circulating  in  the  retina,  and  by  the  little  inter- 
vals of  rest  frequently  occurring.  In  our  experiment  we 
have  merely  confined  the  fatigue  to  one  set  of  nerves,  in- 
stead of  distributing  it  equally  among  the  three  sets. 

The  above  experiments  and  ex])lanation  will  enable  us 


238 


MODERN  CHROMATICS. 


easily  to  comprehend  the  more  complicated  case,  where,  in- 
stead of  placing  our  little  green  square  on  grey,  we  lay  it 
on  a sheet  of  coloured  paper.  Instead,  then,  of  grey,  let  us 
take  yellow  paper,  placing  the  green  square  on  it  as  before. 
Fig.  110.  On  suddenly  withdrawing  the  green  square,  we 
tind  it  replaced  by  an  orange-coloured  ghost.  Fig.  Ill, 


YELLOW 


GREEN 


Fig.  110. — Yellow  Ground  with  Green 
Slip. 


YELLOW 
\ ORANGE  \ 

Fig.  111.— Yellow  Ground  with  Or- 
ange-coloured Image. 


which  we  account  for  thus  : As  before,  the  green  nerves 
are  fatigued,  the  red  and  violet  nerves  remaining  fresh  ; 
when  the  square  is  removed,  yellow  light  is  presented  to 
the  retina,  and  this  yellow  light,  as  explained  in  Chapter 
IX.,  tends  to  act  on  the  red  and  green  nerves  equally  ; but 
the  green  nerves  in  the  present  case  do  not  respond  with 
full  activity,  hence  the  action  is  more  confined  to  the  red 
nerves,  and,  as  explained  in  Chapter  X.,  the  resultant  tint 
is  necessarily  orange — that  is  to  say,  we  have  a strong  red 
sensation  mino;led  with  a weak  green  sensation,  and  the  re- 
suit  is  the  sensation  called  orange.  In  this  experiment  the 
violet  nerves  do  not  come  into  play  to  any  great  extent.  If 
the  green  square  is  placed  on  a blue  ground,  the  image  be- 
comes violet,  for  the  reason  that  the  blue  light  which  is 
presented  to  the  fatigued  retina  acts,  as  explained  in  Chapter 
IX.,  on  the  green  and  violet  nerves  ; but  the  green  nerves 
being  fatigued,  the  action  is  mostly  confined  to  the  violet 
nerves,  and  hence  the  corresponding  sensation.  In  this 
case  the  red  nerves  hardly  come  into  play  at  all. 


CONTRAST. 


239 


It  follows  from  the  above  examples  and  reasoning  that 
the  final  effect  is,  that  we  obtain  as  an  after-image  what 
amounts  to  a mixture  of  the  complementary  colour  of  the 
small  square  with  the  colour  of  the  ground  ; and,  by  recol- 
lecting this,  we  can  easily  retain  this  class  of  facts  in  the 
memory. 

There  is  another  similar  experiment  which  is  simpler 
than  those  just  described,  but  which  is  nevertheless  instruc- 
tive. A small  square  of  black  paper  is  to  be  placed  on  a 
sheet  of  red  paper,  and  the  attention  in  this  case  is  to  be 
directed  to  a mark  on  the  edge  of  the  former.  (See  Fig. 
112.)  When  the  black  square  is  suddenly  removed,  the  ob- 


server sees  in  place  of  it  a more  luminous  spot,  which  in 
the  case  before  us  will  of  course  be  red ; but  what  is  re- 
markable is  the  circumstance  that  this  red  image  will  be 
more  intense  or  saturated  in  colour  than  the  rest  of  the 
ground.  The  rest  of  the  sheet  of  red  paper  will  look  as 
though  grey  had  been  mixed  with  its  colour.  Fig.  113. 
This  experiment  will  of  course  succeed  with  paper  of  any 
bright  colour,  and  Helmholtz  has  found  that  the  same  ef- 
fects can  be  obtained  with  the  pure  colours  of  the  prismatic 
spectrum.  The  explanation,  according  to  our  theory,  runs 
about  thus  : While  we  are  in  the  act  of  looking  at  the  edge 
of  the  black  square,  red  light  is  passing  into  the  eye,  and  is 


^40 


MODERN  CHROMATICS. 


fatiguing  all  those  portions  of  the  retina  that  are  not  pro- 
tected by  the  presence  of  the  black  square  ; it  thus  happens 
that  the  ability  of  the  larger  portion  of  the  retina  to  receive 
the  sensation  of  red  is  considerably  diminished  ; the  ability 
of  the  protected  portion  of  course  suffers  no  such  change. 


"VYhen  the  black  square  is  suddenly  removed,  the  unfatigued 
portion  of  the  retina  receives  a powerful  impulse  from  the 
red  surface,  but  the  effect  produced  on  the  rest  of  the  retina 
is  inferior  in  decree.  This  accounts  for  the  fact  that  the 

O 

image  of  the  square  is  brighter  or  more  luminous  ; and  we 
can  easily  understand  why  it  is  at  the  same  time  more  in- 
tense, or  saturated  in  colour,  if  we  remember,  as  explained 
in  Chapter  IX.,  that  red  light  excites  into  action  not  only 
the  red  nerves,  but  to  a less  extent  the  green  and  violet 
nerves.  Xow,  as  the  red  nerves  begin  to  be  fatigued,  the 
action  of  tlie  other  two  sets  will  be  relatively  more  power- 
ful than  at  first,  so  that  gradually  the  sensations  of  green 
and  violet  begin  to  add  themselves  to  that  of  red  (or,  what 
is  the  same  thing,  the  sensation  of  white  mingles  itself  with 
that  of  red),  and  the  red  colour  of  the  paper  looks  a little 
greyish.  The  success  of  the  experiment  with  the  pure  col- 
ours of  the  prismatic  spectrum,  which  contain  no  white,  is 
easily  accounted  for  by  the  explanation  just  given. 

This  matter  can  be  pushed  even  further  if,  instead  of 


CONTRAST. 


241 


employing  a black  square,  we  take  one  wbicli  has  a colour 
complementary  to  that  of  the  ground.  We  substitute  then 
for  the  black  square  one  coloured  with  emerald-green,  and 
repeat  the  experiment.  (See  Fig.  114.)  The  result  is  much 
the  same  as  before,  except  that  the  red  ghost  is  now  still 
more  intense  or  saturated  in  colour.  Fig.  115.  When  the 


Fig.  114.— Blue-green  Slip  on  Red  Ground.  Fig.  115.— Intense  Red  Image  on  Red 

Ground. 


experiment  is  made  in  this  way  we  accomplish  two  objects  : 
first,  we  protect  a small  portion  of  the  retina  from  red  light, 
so  that  it  may  be  very  sensitive  to  this  kind  of  light  after- 
ward ; second,  we  fatigue  the  green  and  violet  nerves  of 
this  portion  by  presenting  to  them  bluish-green  light,  so 
that  afterward  the  red  light  from  the  red  paper  will  be  un- 
able to  stimulate  them  even  in  a small  degree  ; hence  the 
sensation  that  we  receive  is  that  of  pure  red,  the  action  of 
the  green  and  violet  nerves  being  excluded. 

All  these  phenomena  are  cases  of  what  is  called  succes- 
sive contrast,  because  we  look  in  succession  from  one  sur- 
face to  another.  When  coloured  surfaces  are  placed  near 
each  other  and  compared  in  a natural  manner,  successive 
contrast  plays  an  important  part,  and  the  appearance  of  the 
colours  is  more  or  less  modified  according  to  its  laws.  If 
we  attempt  to  confine  our  attention  to  only  one  of  the  col- 
oured surfaces,  this  still  holds  good  ; for  the  eye  involun- 
tarily wanders  to  the  other,  and  to  prevent  tliis  requires  a 


242 


MODERN  CHROMATICS. 


good  deal  of  careful  practice,  for  fixed  vision  is  quite  op- 
posed to  our  natural  habit.  It  follows  from  this  that,  in 
the  natural  use  of  the  eye,  the  negative  images,  although 
present  to  some  extent,  are  not  sharp  and  distinct,  and 
hence  usually  remain  unobserved  by  persons  not  trained  to 
observations  of  this  character.  Nevertheless  these  images 
modify  to  a considerable  extent  the  appearances  of  coloured 
surfaces  placed  near  each  other,  and  the  changes  of  hue  are 
visible  enough  to  the  most  uneducated  eye. 

One  of  the  most  common  cases  belonging  here  is  repre- 
sented in  Fig.  116.  We  have  a grey  pattern  traced  on  a 


Fig.  116.— Grey  Figiire  on  a Green  Ground. 


green  ground  ; the  tracery,  however,  will  not  appear  pure 
grey,  but  tinged  with  a colour  complementary  to  that  of 
the  ground — that  is,  reddish.  We  can,  of  course,  substi- 
tute for  the  green  any  other  bright  colour,  and  it  will  al- 


CONTKAST. 


243 


ways  be  found  that  the  grey  is  more  or  less  tinged  with  the 
complementary  hue.  As  black  is  really  a dark  grey,  we 
should  expect  to  find  it  also  assuming  to  some  extent  a col- 
our complementary  to  that  of  the  ground  ; and  this  is  in- 
deed the  case,  though  the  effect  is  not  quite  so  marked  as 
with  a grey  of  medium  depth.  Cheyreul,  in  his  great  work 
on  the  simultaneous  contrast  of  colours,  relates  an  anecdote 
which  illustrates  the  matter  now  under  consideration.  Plain 
red,  violet-blue,  and  blue  woven  stuffs  were  given  by  cer- 
tain dealers  to  manufacturers,  with  the  request  that  they 
should  ornament  them  with  black  patterns.  When  the 
goods  were  returned,  the  dealers  complained  that  the  pat- 
terns were  not  black,  maintaining  that  those  traced  on  the 
red  stuffs  were  green,  on  the  violet  dark-greenish  yellow, 
and  on  the  blue  copper-coloured.  Chevreul  covered  the 
ground  with  white  paper  in  such  a way  as  to  expose  only 
the  pattern,  when  it  was  found  that  its  colour  was  truly 
black,  and  the  effects  which  had  been  observed  were  entire- 
ly due  to  contrast.  The  remedy  in  such  cases  is  not  to  em- 
ploy pure  black,  but  to  give  it  a tint  a little  like  that  of  the 
coloured  ground,  taking  care  to  make  it  just  strong  enough 
to  balance  the  hue  generated  by  contrast.  If  we  substitute 
a white  pattern  for  the  black,  something  of  this  same  effect 
can  often  be  observed,  but  it  is  less  marked  than  with  grey 
or  black.  In  cases  like  those  now  under  consideration  the 
contrast  is  stronger  when  the  coloured  surface  is  bright  and 
intense  or  saturated  in  hue.  The  effect  is  also  increased  by 
entirely  surrounding  the  second  colour  with  the  first  ; the 
circumscribing  colour  ought  also  to  be  considerably  larger 
than  its  companion.  When  these  conditions  are  observed, 
the  effect  of  contrast  is  generally  noticeable  only  on  the 
smaller  surface,  the  larger  one  being  scarcely  affected. 

When,  on  the  other  hand,  the  two  coloured  surfaces  are 
about  equal  in  extent,  then  both  suffer  change.  If  it  is  de- 
sired to  produce  a strong  effect  of  contrast,  the  coloured 
surfaces  must  be  placed  as  near  each  other  as  possible. 


244 


MODERN  CHROMATICS. 


This  is  beautifully  illustrated  in  one  of  the  methods  em- 
ployed by  Chevreul  in  studying  the  laws  of  contrast.  Two 
coloured  strips  were  placed  side  by  side  in  contact,  as  shown 
in  Fig.  117,  duplicate  strips  being  arranged  in  the  field  of 


ULTRAMARINE 


CYAN  BLUE 


ULTRAMARINE  CYAN  BLUE 


Fig.  117. — Arrangement  to  show  the  Effects  of  Simultaneous  Contrast,  half  size. 

view  at  some  distance  from  each  other.  The  tints  of  the 
two  central  strips  were  both  altered  ; those  placed  at  a 
greater  distance  apart  suffered  no  change.  In  the  experi- 
ment represented  in  Fig.  117  the  central  ultramarine  by 
contrast  is  made  to  appear  more  violet  in  hue,  the  central 
cyan-blue  more  greenish  ; the  colour  of  the  outlying  strips 
is  scarcely  aft’ected.  In  this  experiment  we  have  an  appli- 
cation of  the  rule  above  given  for  determining  the  changes 
which  colours  experience  under  the  influence  of  contrast. 
The  rule  is  quite  simple  ; its  application,  however,  involves 
a knowledge  of  the  colours  which  are  complementary  to 
each  other,  as  well  as  of  the  effects  produced  by  mixing  to- 
gether masses  of  coloured  light.  According  to  our  rule, 
when  two  coloured  surfaces  are  placed  in  contiguity,  each 
is  changed  as  though  it  had  been  mixed  to  some  extent 
with  the  complementary  colour  of  the  other.  In  the  exam- 
ple before  us  the  ultramarine  becomes  more  of  a violet-blue, 


CONTRAST. 


245 


because  it  is  mixed,  or  seems  to  be  mixed,  with  the  comple- 
mentary colour  of  cyan-blue — that  is,  with  orange.  The 
cyan-blue  appears  more  greenish,  because  it  is  virtually 
mixed  with  greenish-yellow,  which  is  the  complementary 
colour  of  ultramarine.  As  it  requires  a little  consideration 
to  predict  the  changes  which  colours  undergo  through  con- 
trast, we  give  below  a table  containing  the  most  important 
cases  ; 


Pairs  of  Colours. 

Change  due  to  Contrast, 

^ Red 

} Orange 

^ Red 

( Yellow 

j Red . . . . 

( Blue-green 

“ “ brilliant. 

{ Red 

( Blue 

< Red 

1 Violet 

j Orange 

“ “ red-orange. 

i Yellow 

“ greenish-yellow. 

j Orange 

“ “ red-orange. 

( Green 

1 Orange 

“ “ brilliant. 

} Cyan-blue 

. ...  “ “ brilliant. 

i Orange 

“ “ yellowish. 

/ Violet 

“ “ bluish. 

{ Yellow 

“ “ orange-yellow. 

] Green 

“ “ bluish-green. 

\ Yellow 

“ “ orange-yellow. 

f Cyan-blue 

....  “■  “ blue. 

{ Yellow 

“ “ brilliant. 

( Ultramarine-blue 

“ “ brilliant. 

i Green 

“ “ yellowish-green. 

f Blue 

“ “ purplish. 

j Green 

“ “ yellowish-green. 

( Violet 

j Greenish-yellow  

...  “ “ brilliant. 

i Violet 

“ “ brilliant. 

(Blue 

“ “ greenish. 

( Violet 

“ “ purplish. 

246 


MODERN  CHROMATICS. 


It  is  easy  and  instructive  to  study  the  changes  produced  by 
contrast  Avith  the  aid  of  a chromatic  circle,  Fig.  118,  and  it 


Fig.  1 is.— Chromatic  Circle. 


will  he  found  that  alterations  in  colour  produced  by  con- 
trast obey  a very  simple  law  : When  any  two  colours  of 
the  chromatic  circle  are  brought  into  competition  or  con- 
trasted, the  etfect  ]>roduced  is  apparently  to  move  them 
both  farther  a])art.  In  the  case,  for  example,  of  orange 
and  yellow,  the  orange  is  moved  toward  the  red,  and  as- 
sumes the  appearance  of  reddish-orange  ; the  yellow  moves 
toward  the  green,  and  appears  for  the  time  to  be  greenish- 
yellow.  Colours  Avhich  are  complementary  are  already  as 
far  apart  in  the  chromatic  circle  as  possible  ; hence  they  are 
not  changed  in  hue,  but  merely  appear  more  brilliant  and 
saturated.  This  is  indeed  the  effect  Avhich  a strict  applica- 
tion of  our  rule  leads  to  : the  tAvo  colours  are  to  be  moved 
farther  apart  ; they  are  already  situated  on  the  opposite 
extremities  of  a diameter  of  the  circle,  and,  if  they  are  to 
recede  still  farther  from  each  other,  they  can  accomplish 
this  in  no  other  way  than  by  moving  outside  of  the  circum- 
ference of  the  circle  ; but  this  corresponds,  as  explained  in 
the  previous  chapter,  to  an  increase  of  saturation.  If  the 


CONTKAST. 


247 


experiments  indicated  in  the  table  are  carefully  repeated,  it 
will  be  found  that  all  the  pairs  of  colours  there  enumerated 
are  not  equally  affected  by  contrast.  The  changes  of  tint 
are  greatest  with  the  colours  which  arje  situated  nearest  to 
each  other  in  the  chromatic  circle,  and  much  less  with  those 
at  a distance.  Thus  both  red  and  yellow  are  much  changed 
by  contrast,  the  red  becoming  pu;rplish,  the  yellow  greenish  ; 
while  red  with  cyan-blue  or  blue  is  much  less  affected  in 
the  matter  of  displacement  or  change  of  hue.  On  the  other 
hand,  the  colours  which  are  distant  from  each  other  in  the 
chromatic  circle,  while  suffering  but  slight  changes  in  hue, 
are  made  to  appear  more  brilliant  and  saturated  ; that  is, 
they  are  virtually  moved  somewhat  outside  of  the  circle, 
the  maximum  effect  taking  place  with  colours  which  are 
complementary. 

Colours  which  are  identical  are  affected  by  contrast  in 
exactly  the  opposite  way  from  those  which  are  complemen- 
tary ; that  is,  they  are  made  to  appear  duller  and  less  satu- 
rated. The  author  finds  that  these  and  other  effects  of 
contrast  can  be  studied  with  great  advantage  by  the  aid  of 
two  identical  chromatic  circles  laid  down  on  paper.  One 
set  of  these  lines  should  be  traced  on  a sheet  of  transparent 
paper,  which  is  afterward  to  be  placed  over  the  companion 
circle.  The  use  of  these  circles  will  best  be  made  evident 
with  the  aid  of  an  example.  Let  us  suppose  that  we  wish 
to  ascertain  with  their  aid  the  effect  produced  by  red,  as 
far  as  contrast  goes,  on  all  the  other  colours,  and  also  on 
red  itself.  We  place  the  transparent  circle  on  its  compan- 
ion, so  that  the  two  drawings  may  coincide  in  position, 
and  we  then  move  the  upper  circle  along  the  diameter  join- 
ing red  and  green-blue  some  little  distance,  so  that  the  two 
circles  no  longer  have  the  same  common  centre.  We  then 
transfer  the  points  marked  red,  orange,  yellow,  etc.,  on  the 
upper  circle,  by  pricking  with  a pin,  to  the  lower  circle,  and 
these  pin-marks  on  the  lower  circle  will  indicate  the  changes 
produced  on  all  the  colours  by  compeiition  with  red.  Fig. 


248 


MODERN  CHROMATICS. 


119  gives  the  result.  The  stars  on  the  dotted  circle  repre- 
sent the  new  positions  of  the  different  colours  when  con- 
trasted with  red.  If  we  examine  them  we  find  that  red 
when  contrasted  with  greenish-blue  causes  this  last  colour 
to  move  away  from  the  centre  of  the  circle  in  a straight 
line  ; hence,  as  the  new  point  is  on  the  same  diameter,  but 
farther  from  the  centre,  Ave  know  that  the  greenish-blue  is 


Fig.  Chromatic  Circle  displaced  by  Contrast,  showing  the  effects  produced  by  the 
red  on  the  other  colours. 

not  made  more  or  less  blue  or  green,  but  is  simply  caused 
to  appear  more  saturated  or  brilliant.  The  new  point  for 
the  red  lies  also  on  the  same  diameter,  but  is  nearer  to  the 
centre  of  the  circle  ; that  is,  the  colour  remains  red,  but 
appears  duller  or  less  saturated.  Experience  confirms  this. 
If  a considerable  number  of  pieces  of  red  cloth,  for  exam- 
ple, are  examined  in  succession,  the  last  one  will  appear 
duller  and  inferior  in  brilliancy  to  the  others,  but  it  will 
still  appear  red.  Proceeding  with  the  examination  of  the 
effects  produced  on  the  other  colours,  we  find  that  orange 
has  been  moved  toward  yellow,  and  also.  toAvard  the  centre 
of  the  circle  ; hence  our  diagram  tells  us  that  red,  when  put 
into  competition  with  orange,  causes  the  latter  to  appear 


CONTRAST. 


249 


more  yellowish  and  at  the  same  time  less  intense.  Advanc- 
ing along  the  circumference  of  the  circle,  our  diagram  in- 
forms us  that  yellow  is  not  much  affected  in  the  matter  of 
saturation  or  intensity,  hut  is  simply  made  to  appear  more 
greenish.  The  two  circles  during  superposition  cut  each 
other  near  the  position  of  yellow  ; from  this  point  onward 
the  effect  changes  as  far  as  intepsity  or  saturation  is  con- 
cerned, the  greenish-yellow  being  moved  decidedly  outside 
of  the  original  circle,  as  well  as  tow^ard  the  green  ; it  is 
made  therefore,  by  contrast  with  red,  to  appear  more  bril- 
liant as  well  as  more  greenish.  Green  is  made  to  appear 
somewhat  bluish,  and  more  brilliant.  Greenish-blue  has 
been  considered.  Cyan-blue  is  made  to  appear  slightly 
more  greenish  as  well  as  much  more  brilliant ; the  same  is 
true  of  blue,  though  its  increase  in  brilliancy  by  contrast 
with  red  is  rather  less  than  in  the  case  with  cyan-blue. 
Violet  has  its  hue  considerably  altered  toward  blue  ; its 
saturation  is  diminished.  Purple  is  made  to  look  more  violet, 
and  is  much  diminished  in  saturation.  If  we  wish  to  study 
the  effects  produced  on  the  colours  of  the  chromatic  circle 
by  contrasting  them  with  yellow,  we  have  of  course  merely 
to  displace  the  upper  circle  along  the  line  joining  yellow 
and  its  complement  ultramarine-blue,  and  then  proceed  as 
before.  The  proper  amount  of  displacement  will  of  course 
not  be  very  large,  and  can  be  approximately  determined  by 
experiment  ; the  upper  circle,  namely,  is  to  be  moved,  so 
that  the  colours  situated  on  either  side  of  the  points  where 
the  circles  cut  each  other  shall,  in  the  diagram.  Fig.  118,  be 
made  to  suffer  changes  of  saturation  corresponding  to  the 
results  of  actual  experiment. 

It  is  quite  evident  that  this  contrast-diagram  will  fur- 
nish correct  results  only  on  condition  that  the  colours  in  it 
are  properly  arranged  ; if  the  angular  positions  of  the  col- 
ours are  laid  down  falsely,  the  results,  in  the  matter  of  in- 
crease or  diminution  of  brilliancy  or  saturation,  will  also  be 
false.  The  author  has  made  many  experiments  to  settle 


250 


MODERN  CHROMATICS. 


4 

this  question,  and  in  Fig.  120  gives  his  result  in  the  form 
of  a diagram  ; the  same  result  is  given  below  in  the  form 
of  a table  : 


Table  showing  the  Distances  of  the  Colours  from  each  other  in  the 
Contrast-Circle,  according  to  0.  N.  R. 


Angular  Distances. 

Pure  red  to  vermilion 6° 

Vermilion  to  red  lead 10° 

Red  lead  to  orange 0° 

Orange  to  orange-yellow 36° 

Orange-yellow  to  yellow 28° 

Yellow  to  greenish-yellow 23° 

Greenish-yellow  to  yellowish-green • 13° 

Yellowish-green  to  green.  22° 


CONTRAST. 


251 


Green  to  emerald-gree^. . . 10° 

Emerald-green  to  very  greenish  blue,  or  to  complement  of 
carmine 18° 

The  hues  of  the  papers  employed  in  these  experiments  were 
determined  with  some  degree  of  accuracy  by  comparing 
them  with  a normal  spectrum  nearly  six  times  as  long  as 
that  furnished  by  a single  flint-^glass  prism,  and  at  the  same 
time  brilliant  and  pure.  (See  Chapter  III.)  The  following 
table  gives  the  positions  of  these  coloured  papers  in  a nor- 
mal spectrum,  containing  from  A to  H 1,000  equal  parts  ; 
the  corresponding  wave-lengths  are  also  given  : 


Coloured  Papers. 

Position  in  Nor- 
mal Spectrum. 

Wave-length  in 
To  00^6  0 0 rnm. 

Spectral  red  (vermilion  washed  with  carmine) 
Vermilion  (English  1 .• 

285 

387 

6562 

6290 

Red  lead 

422 

6061 

Orange 

448 

6000 

Yellow  (pale  chrome) 

488 

6820 

Greenish-yellov,^ 

535 

5649 

Yellow-green 

552 

5587 

Green 

1 

600 

5411 

Einerald-"Tecn 

648 

6236 

Cyan-blue  2 

715 

4991 

Ultramarine,  natural 

785 

4735 

Ultramarine,  artificial 

857 

4472 

Violet  (“  IIolFinann’s  violet  B.  B.”) Rather  more  reddish  than 

any  violet  in  the  spectrum. 


From  the  foregoing,  then,  it  is  evident  in  general  that 
the  effect  of  contrast  may  be  helpful  or  harmful  to  colours  : 
by  it  they  may  be  made  to  look  more  beautiful  and  pre- 
cious, or  they  may  damage  each  other,  and  then  appear 
<lull,  pale,  or  even  dirty.  When  the  apparent  saturation  is 
increased,  we  have  the  first  effect  ; the  second,  when  it  is 
diminished.  Our  diagram.  Fig.  110,  shows  that  the  salura- 


252 


MODERN  CRHOMATICS. 


tion  is  diminished  when  the  contrasting  colours  are  situated 
near  each  other  in  the  chromatic  circle,  and  increased  when 
the  reverse  is  true.  It  might  be  supposed  that  we  could 
easily  overcome  the  damaging  effects  of  harmful  contrast 
by  simply  making  the  colours  themselves  from  the  start 
somewhat  more  brilliant  ; this,  however,  is  far  from  being 
true.  The  pleasure  due  to  helpful  contrast  is  not  merely 
owing  to  the  fact  that  the  colours  appear  brilliant  or  satu- 
rated, but  that  they  have  been  so  disposed,  and  provided 
with  such  companions,  that  they  are  made  to  glow  with 
more  than  their  natural  brilliancy.  Then  they  strike  us  as 
precious  and  delicious,  and  this  is  true  even  when  the  actual 
tints  are  such  as  we  would  call  poor  or  dull  in  isolation. 
From  this  it  follows  that  paintings,  made  up  almost  entirely 
of  tints  that  by  themselves  seem  modest  and  far  from  bril- 
liant, often  strike  us  as  being  rich  and  gorgeous  in  colour  ; 
while,  on  the  other  hand,  the  most  gaudy  colours  can  easily 
be  arranged  so  as  to  produce  a depressing  effect  on  the  be- 
holder. We  shall  see  hereafter  that,  in  making  chromatic 
compositions  for  decorative  purposes  or  for  paintings,  artists 
of  all  times  have  necessarily  been  controlled  to  a consider- 
able extent  by  the  laivs  of  contrast,  which  they  have  in- 
stinctively obeyed,  just  as  children  in  walking  and  leaping 
respect  the  law  of  gravitation,  though  unconscious  of  its 
existence. 

The  phenomena  of  contrast,  as  exhibited  by  colours 
which  are  intense,  pure,  and  brilliant,  are  to  be  explained 
to  a considerable  extent  by  the  fatigue  of  the  nerves,  as 
set  forth  in  the  early  part  of  the  present  chapter.  The 
changes  in  colour  and  saturation  become  particularly  con- 
spicuous after  somewhat  prolonged  observation,  and  are 
often  attended  with  a peculiar  soft  glimmering,  which 
seems  to  float  over  the  surfaces,  and,  in  the  case  of  colours 
that  are  far  apart  in  the  chromatic  circle,  to  lend  them  a 
lustrous  appearance.  Still,  upon  the  whole,  the  effects  of 
contrast  with  brilliant  colours  are  often  not  strongly  marked 


CONTRAST. 


253 


at  first  glance,  from  the  circumstance  that  the  colours,  by 
virtue  of  their  actual  intensity  and  strength,  are  able  to 
resist  these  changes,  and  it  often  requires  a practised  eye 
to  detect  them  with  certainty.  The  case  is  quite  other- 
wise with  colours  which  are  more  or  less  pale  or  dark — that 
is,  which  are  deficient  in  saturation  or  luminosity,  or  both. 
Here  the  original  sensation  produced  on  the  eye  is  compara- 
tively feeble,  and  it  is  hence  more  readily  modified  by  con- 
trast. In  these  cases  the  fatigue  of  the  nerves  of  the  retina 
plays  but  a very  subordinate  part,  as  we  recognize  the  ef- 
fects of  contrast  at  the  first  glance.  We  have  to  deal  here 
with  what  is  known  as  simultaneous  contrast,  the  effects 
taking  place  when  the  two  surfaces  are  as  far  as  possible 
regarded  simultaneously.  In  the  case  of  simultaneous  con- 
trast the  changes  are  due  mainly  to  fluctuations  of  the 
judgment  of  the  observer,  but  little  to  the  fatigue  of  the 
retinal  nerves. 

We  carry  in  ourselves  no  standard  by  which  we  can 
measure  the  saturation  of  colour  or  its  exact  place  in  the 
chromatic  scale  ; hence,  if  we  have  no  undoubted  external 
standard  at  hand  with  which  to  compare  our  colours,  we 
are  easily  deceived.  A slip  of  paper  of  a pale  but  very 
decided  blue-green  hue  was  placed  on  a sheet  of  paper  of 
the  same  general  tint,  but  somewhat  darker  and  more  in- 
tense or  saturated  in  hue.  The  small  slip  now  appeared 
pure  grey,  and  by  no  effort  of  the  reason  or  imagination 
could  it  be  made  to  look  otherwise.  In  this  experiment  no 
undoubted  pure  grey  was  present  in  the  field  of  view  for 
comparison,  and  in  point  of  fact  the  small  slip  did  actually 
approach  a pure  grey  in  hue  more  nearly  than  the  large 
sheet ; hence  the  eye  instantly  accepted  it  for  pure  grey. 
The  matter  did  not,  however,  stop  here.  A slip  of  pure 
grey  paper  was  now  brought  into  the  same  green  field,  but, 
instead  of  serving  as  a standard  to  correct  the  illusion,  it 
assumed  at  once  the  appearance  of  a redclish-gvQy.  The 
pure  grey  really  did  approach  reddish-grey  more  than  the 


254 


MODERN  CHROMATICS. 


green  field  surrounding  it,  and  hence  was  accepted  for  this 
tint.  The  same  pale  blue-green  slip,  when  placed  on  a pale- 
reddish  ground,  assumed  a stronger  blue-green  hue  than 
when  on  a white  ground.  In  the  first  of  these  experiments] 
we  have  an  illustration  of  harmful  and  in  the  second  of' 
helpful  simultaneous  contrast.  The  result  in  both  cases  co- 
incided with  that  Avhich  .successive  contrast  would  have 
produced  under  similar  circumstances. 

It  has  been  stated  above  that  the  effects  produced  by 
simultaneous  contrast  are  due  not  to  retinal  fatigue,  but  to 
deception  of  the  judgment  ; now,  as  the  effects  of  simulta- 
neous contrast  are  identical  in  kind  with  those  generated  by 


Fig.  121.— Shadow  of  Kod  in  Darkened  Eoom. 


successive  contrast,  it  is  evident  that  the  statement  needs 
some  proof.  This  can  be  furnished  with  the  aid  of  a beau- 
tiful experiment  with  coloured  shadows.  In  making  this 
experiment  Ave  allow  Avhite  daylight  to  enter  a darkened 
room  through  an  aperture.  A,  arranged  in  a window,  as  in- 
dicated in  Fig.  12 1.  At  R Ave  set  up  a rod,  and  allow  its 
shadoAv  to  fall  on  a sheet  of  Avhite  cardboard,  or  on  the 


CONTRAST. 


255 


white  wall  of  the  room.  It  is  evident  now  that  the  whole 
of  the  cardboard  will- be  illuminated  with  white  light,  ex- 
cept those  portions  occupied  by  the  shadow  1.  We  then 
light  the  candle  at  C,  Fig.  122  ; its  light  will  also  fall  on 


Fig.  122.— Shadows  of  Eod,  using  Daylight  and  Candle-light. 


the  cardboard  screen,  and  will  then  cast  the  shadow  2 ; that 
is,  the  candle-light  will  illuminate  all  parts  of  the  screen 
except  those  occupied  by  the  shadow  2 ; this  portion  will 
be  illuminated  with  pure  white  light.  Instead,  however,  of 
appearing  to  the  eye  white,  the  shadow  2 will  seem  to  be 
coloured  decidedly  blue.  For  the  production  of  the  most 
powerful  effect,  it  is  desirable  that  the  shadows  should 
have  the  same  depth,  which  can  be  effected  by  regulating 
the  size  of  the  aperture  admitting  daylight.  Now,  although 
the  shadow  cast  by  the  candle  is  actually  pure  white,  yet, 
by  contrast  with  the  surrounding  orange-yellow  ground,  it 
is  made  to  appear  decidedly  blue.  So  strong  is  the  illusion 
that,  even  after  the  causes  which  gave  rise  to  it  have  disap- 
])earcd,  it  still  persists,  as  can  be  shown  by  the  following 
experiment  of  Helmholtz;  While  the  coloured  shadows  are 


256 


MODERN  CHROMATICS. 


falling  on  the  screen,  they  are  to  be  viewed  through  a 
blackened  tube  of  cardboard,  held  in  such  a way  that  the 
observer  has  both  the  shadows  in  his  field  of  view  ; the  ap- 
pearance then  will  be  like  that  represented  in  Fig.  123. 


Fig.  123.— Blue  and  Yellow  Shadows  viewed  through  a Tube. 


After  the  blue  shadow  has  developed  itself  in  full  intensity, 
the  tube  is  to  be  moved  to  the  left,  so  that  the  blue  shadow 
may  fill  the  whole  field.  The  tube  being  held  steadily  in 
the  new  position,  the  shadow  will  still  continue  to  appear 
blue  instead  of  white,  even  although  the  exciting  cause,  viz., 
the  orange-yellow  candle-light,  is  no  longer  acting  on  the 
eye.  The  candle  may  be  blown  out,  but  the  surface  will 
still  appear  blue  as  long  as  the  eye  is  at  the  tube.  On  re- 
moving the  tube,  the  illusion  instantly  vanishes,  and  it  is 
perceived  that  the  colour  of  the  surface  is  identical  with 
that  of  the  rest  of  the  screen,  which  is  at  once  recognized 
as  white.  In  a case  like  this  the  fatigue  of  the  retinal  ele- 
ments can  play  no  part,  as  the  illusion  persists  during  a far 
longer  period  of  time  than  is  necessary  for  their  conij^lete 
rest  ; we  must  hence  attribute  the  result  to  a deception  of 
the  judgment.  Expressing  this  in  the  language  of  Young’s 
theory,  we  say  that  the  sensation  of  white  is  produced 
when  the  three  sets  of  nerves,  red,  green,  and  violet,  are 


CONTRAST. 


257 


Stimulated  to  about  the  same  extent ; but  that  nevertheless, 
as  we  have  in  ourselves  no  means  of  judging  with  certainty 
about  this  equality  of  stimulation,  we  may  under  certain 
circumstances  be  induced  to  accept  an  unequal  for  an  equal 
stimulation,  or  the  reverse.  In  the  experiment  with  the 
coloured  shadows  we  had  before  us  in  the  shadow  due  to 
the  candle-flame  an  equal  stiniulation,  which  by  contrast 
we  were  in  the  first  instance  induced  to  accept  as  unequal, 
and  the  judgment  afterward  obstinately  persisted  in  the 
error  till  it  was  corrected  and  took  a new  departure. 

This  experiment  may  be  modified  and  extended  by  the 
use  of  coloured  glasses  instead  of  a candle-flame.  The  win- 
dow is  to  be  provided  with  two  apertures,  one  of  which  is 
to  be  covered  with  a piece  of  stained  glass,  through  which 
sunshine  will  be  admitted  to  the  darkened  room  ; the  other 
aperture  will  admit  white  light,  as  before.  If  red  glass  be 
employed,  the  colour  of  the  shadows  will  appear  red  and 
greenish-blue.  In  each  case  the  shadows  will  assume  com- 
plementary colours. 

The  effects  of  simultaneous  contrast  can  also  be  studied 
with  the  aid  of  a contrivance  of  Ragona  Scina.  Two  sheets 
of  white  cardboard  are  attached  to  a couple  of  boards  fast- 
ened together  at  a right  angle,  as  indicated  in  Fig.  124. 
Between  the  boards  a plate  of  rather  deeply  coloured  glass, 
G,  is  to  be  held  in  the  manner  shown  in  the  figure,  so  that 
it  makes,  with  the  vertical  and  horizontal  cardboards,  an 
angle  of  about  45°.  If  the  eye  is  placed  at  E,  two  masses 
of  light  will  be  sent  to  it.  From  the  vertical  cardboard 
white  light  will  start,  and,  after  being  reflected  on  the  glass 
plate  G,  will  reach  the  eye.  This  light  will  be  white,  or 
almost  entirely  white,  even  after  suffering  reflection,  owing 
to  the  circumstance  that,  with  a deeply  coloured  plate  of 
glass,  the  reflection  takes  place  almost  entirely  from  the 
upper  surface,  or  from  that  turned  toward  the  light.  Tlie 
second  mass  of  light  will  proceed  from  the  horizontal  plate 
II  : originally  of  course  it  was  white  light,  but  on  its  way 


258 


MODERN  CHROMATICS. 


to  the  eye  it  traverses  the  glass  plate,  and  becomes  coloured 
by  absorption.  If  the  glass  plate  is  red,  this  light  when  it 
reaches  the  eye  will  of  course  have  the  same  colour  ; conse- 
quently the  first  result  is  that  we  have  presented  to  the  eye 


Fig.  124.— Apparatus  of  Eagona  Scina  for  Contrast. 


a mixture  of  red  with  white  light,  which  will  give  the 
observer  the  idea  that  he  is  looking  at  an  horizontal,  square 
field  of  a somewhat  pale  reddish  tint.  If  now  a small 
black  square  be  attached  to  tlie  vertical  cardboard  at  B,  of 
course  no  white  light  can  come  to  the  eye  from  this  portion 
of  the  cardboard,  and  the  image  of  this  spot  will  seem  to 
the  eye  to  be  at  h,  on  the  horizontal  board  under  the  eye. 
Under  ordinary  circumstances  this  image  would  appear 
black  ; in  point  of  fact,  however,  in  this  case  it  appears 
deep  red,  owing  to  the  red  light  transmitted  by  the  plate 
of  o’lass.  Thus  far  the  arrangement  amounts  to  a device 
for  presenting  to  the  eye  a mixture  of  red  with  white  light, 
the  white  light  being  absent  at  a certain  spot,  which  conse- 
quently appears  of  a deeper  red.  A similar  black  square 
is  now  to  be  placed  on  the  horizontal  board  at  c ; it  will  of 


•CONTRAST. 


259 


course  prevent  the  light  from  the  place  it  covers  from 
reaching  either  the  red  glass  or  the  eye,  and  under  ordinary 
circumstances  would  he  perceived  simply  as  a square  black 
spot.  Owing,  however,  to  the  fact  that  the  upper  surface 
of  the  glass  plate  is  reflecting  white  light  to  the  eye,  it 
really  appears  as  a grey  spot.  The  final  result  is,  that  we 
present,  to  the  eye  at  E a pi(?ture  like  that  indicated  by 
Fig.  125  ; that  is,  on  a pale-red  ground  we  have  a spot 
which  is  pure  grey,  and  near  it  one  which  is  deep  red. 


PALE  BED 


RED 


GREY 


Fig.  125.— Colours  that  are  really  presented  to  the  eye  in  the  experiment  of  Ragona 

Scina, 

Owing  to  contrast,  however,  the  ajipearance  is  different  : 
instead  of  a grey  spot,  we  see  one  strongly  coloured  green - 
blue  (Fig.  126).  This  effect  is  partly  due  to  contrast  with 


PALE  RED 


GREEN 

BLUE 


Fig.  120.— Colours  that  are  apparently  presented  to  tho  eye. 


RED 


tlie  ])ale  red  of  the  ground,  but  still  more  to  tlie  jiresence 
of  the  deep-red  spot.  This  latter  we  can  remove  by  taking 
away  the  black  square  B,  which  diminishes  the  effect  con- 


260 


MODERN  CHROMATICS. 


siderably.  But  now  comes  the  most  curious  part  of  this 
experiment : If  we  select  a square  of  grey  paper  which  has 
the  same  color  with  the  grey  square  seen  in  the  apparatus 
arranged  as  in  Fig.  124  (apart  from  effects  of  contrast),  and 
place  it  over  the  glass  plate  and  near  the  other  two  images, 
it  will  not  he  affected  in  colour,  or  only  to  a slight  extent. 
In  point  of  fact,  we  now  have,  side  by  side,  on  the  same 
held,  two  grey  squares  quite  identical  in  actual  colour,  but 
one  appears  by  contrast  blue-green,  while  the  other  is  not 
affected,  but  is  perceived  by  the  eye  as  being  simply  a 
square  of  grey  paper.  As  soon,  however,  as  the  observer 
recognizes  the  fact  that  these  two  squares  really  have  the 
same  grey  colour,  the  illusion  instantly  vanishes,  and  both 
of  them  remain  persistently  grey.  It  is  evident  that  in 
this  case,  as  with  the  coloured  shadows,  the  judgment  is  at 
fault  rather  than  the  retinal  nerves  ; for,  as  soon  as  an 
opportunity  otters,  it  corrects  itself  and  takes  a new  de- 
parture. The  illusion  in  this  case,  as  well  as  with  the  col- 
oured shadows,  is  produced  quite  independently  of  the 
knowledge  of  the  observer,  who  may  indeed  be  a trained 
physicist,  minutely  acquainted  with  the  exact  facts  of  the 
case,  and  with  all  the  details  employed  in  producing  the 
deception,  and  still  find  himself  quite  unable  to  escape  from 
its  enthrallment. 

The  simple  experiments  of  II.  Mayer  are  less  trouble- 
some than  those  just  described,  and  at  the  same  time  highly 
instructive.  A small  strip  of  grey  paper  is  placed  on  a 
sheet  of  green  paper,  as  indicated  in  Fig.  127  ; it  will  be 
found  that  the  tint  of  the  grey  paper  scarcely  changes,  un- 
less the  experimenter  sits  and  stares  at  the  combination 
for  some  time.  A sheet  of  thin  semi-transparent  white 
paper  is  now  to  be  placed  over  the  whole,  when  it  will  in- 
stantly be  perceived  that  the  colour  of  the  small  slip  has 
been  converted  by  contrast  into  a pale  red.  Persons  seeing 
this  illusion  for  the  first  time  are  always  much  astonished. 
Here  we  have  an  experiment  showing  that  the  contrast 


CONTRAST. 


261 


produced  by  strong,  saturated  tints  is  much  feebler  than 
with  tints  which  are  pale  or  mixed  with  much  white  light ; 
for,  by  placing  tissue  paper  over  the  green  sheet,  the  colour 
of  the  latter  is  extraordinarily  weakened  and  mixed  with  a 


GREEN/ 


GREY 


Fig.  127.— Green  and  Grey  Papers,  for  Experiment  on  Contrast ; one-fourth  size. 

large  quantity  of  white  light.  In  this  experiment  it  often 
happens  that  the  red,  which  is  due  to  contrast  alone,  seems 
actually  stronger  than  the  green  ground  itself.  If,  instead 
of  using  a slip  of  grey  paper,  we  employ  one  of  black,  the 
contrast  is  less  marked,  and  still  less  with  one  of  white. 
It  is  scarcely  necessary  to  add  that,  if  red  paper  is  em- 
ployed, the  small  grey  slip  becomes  tinted  by  contrast 
with  the  complementary  colour,  i.  e.,  greenish-blue  ; the 
same  is  true  with  the  other  colours. 

By  preparing  with  Indian  ink  a series  of  slips  of  grey 
paper,  ranging  from  pure  white  to  black,  an  interesting 
series  of  observations  can  be  made  on  the  conditions  most 
favourable  for  the  production  of  strong  contrast-colours. 
The  strongest  contrast  will  be  produced  in  the  case  of  red, 
orange,  and  yellow,  when  the  grey  slip  is  a little  darker 
than  the  colour  on  which  it  is  placed,  the  reverse  being 
true  of  green,  blue,  violet,  and  purple  ; in  every  case  the 
contrast  is  weaker  if  the  grey  slip  is  much  lighter  or  much 
darker  than  the  ground.  We  must  expect  then,  in  paint- 
ing, to  find  that  neutral  grey  will  be  more  altered  by  pale 


262 


MODERN  CHROMATICS. 


tints  of  red,  orange,  or  yellow,  which  are  slightly  lighter 
than  itself,  and  that  the  grey  will  he  less  altered  by  these 
colours  when  differing  considerably  from  it  in  luminosity. 
An  analogous  conclusion  with  regard  to  green,  blue,  violet, 
and  purple  can  also  be  drawn  ; these  colours  should  be 
darker  than  the  grey  slip.  Saturated  or  intense  colours  in 
a painting  have  less  effect  on  white  or  grey  than  colours  that 
are  pale.  This  was  shown  in  the  preliminary  experiment, 
where  grey  was  placed  on  a ground  of  strong  colour.  In 
repeating  these  experiments,  it  will  be  noticed  that  the 
effect  of  contrast  is  stronger  with  green,  blue,  and  violet 
than  with  red,  orange,  or  yellow  ; that  is  to  say,  it  is 
strono-er  with  the  cold  than  with  the  warm  colours.  If 
now  we  reverse  our  mode  of  proceeding,  and  place  a small 
coloured  slip  on  a grey  ground,  and  cover  the  whole  with 
tissue  paper,  it  will  be  difficult  even  with  a green  slip  to 
observe  any  effect  of  contrast.  With  a green  slip,  one 
sometimes  imagines  that  the  white  sheet  looks  slightly 
pinkish  or  purplish  for  an  instant,  but  the  effect  is  quite 
uncertain.  This  is  another  illustration  of  the  fact  that,  for 
the  production  of  strong  effects  of  contrast,  it  is  necessary 
that  the  active  colour  should  have  a surface  considerably 
larger  than  the  one  to  be  acted  on  ; the  former  ought  also, 
if  possible,  to  surround  the  latter.  There  is,  however,  a 
limit  beyond  which  this  can  not  be  carried.  If  the  smaller 
held  is  reduced  too  much  in  size,  it  is  liable  to  melt  to  a 
certain  extent  into  the  larger  field  of  colour,  in  which  case 
we  obtain,  not  the  effect  due  to  contrast,  but  that  pro- 
duced by  a mixture  in  the  eye  of  the  two  colours  ; this  is 
indeed  one  method  employed  by  artists  for  the  mixing  of 
their  colours. 

Leaving  now  the  contrast  between  pale  colours  and 
pure  grey,  we  pause  to  consider  for  a moment  the  contrast 
of  pale  colours  with  each  other.  The  laws  governing  this 
species  of  contrast  have  already  been  explained  in  detail  in 
an  earlier  portion  of  the  present  chapter,  and  a construction 


CONTRAST. 


263 


has  been  given  by  which  the  reader  can  study  the  differ- 
ences produced  in  hue  and  saturation.  To  this  it  may  now 
be  added  that,  where  pale  tints  are  used  in  juxtaposition, 
the  phenomena  are  those  of  simultaneous  contrast,  the  ret- 
inal elements  experiencing  scarcely  any  fatigue  ; hence  the 
effects  are  due  to  deceptions  of  the  judgment,  and  occur 
instantly.  They  are  more  marked  than  with  intense  or  satu- 
rated colours,  and  the  effects  produced  even  much  more  sur- 
prising. These  effects  are  heightened  if  the  contrasted  col- 
ours have  about  the  same  degree  of  luminosity,  or  differ  in 
the  same  manner  as  in  the  spectrum  ; that  is,  if  the  warm 
colours  are  selected  so  as  to  be  rather  more  luminous  than 
those  that  are  cold.  In  Chapter  III.  the  reader  will  find  a 
table  showing  the  relative  luminosity  of  the  different  col- 
ours of  the  spectrum,  and,  what  is  still  more  to  our  purpose, 
another  giving  the  relative  luminosity  of  the  different  com- 
ponents of  white  light. 

We  must  next  examine  the  effects  that  are  produced  by 
contrasting  colours  that  differ  in  luminosity  or  in  satura- 
tion. If  the  two  colours  are  identical  except  in  the  matter 
of  saturation,  it  will  be  found  that  the  one  which  is  more 
saturated  will  gain  in  intensity,  while  its  pale  rival  will 
appear  still  paler.  A slip  of  paper  painted  with  a some- 
what pale  red,  when  placed  on  a vermilion  ground,  appears 
still  paler,  and  may  actually  be  made  to  look  white.  If  a 
still  paler  slip  be  used,  it  may  even  become  tinged  greenish- 
blue,  its  colour  being  in  this  case  actually  reversed  by  the 
effect  of  contrast.  With  the  aid  of  the  two  movable  chro- 
matic circles  shown  in  Fig.  119,  it  is  easy  to  trace  these 
changes  theoretically.  As  a pale  colour,  or  one  mixed  with 
much  white,  already  lies  near  the  centre  of  the  upper  cir- 
cle,* a small  displacement  carries  it  to  the  centre,  that  is, 
makes  it  appear  white  ; or  it  may  even  transport  it  beyond, 
and  cause  it  to  assume  the  com})lenientary  colour.  When 


12 


See  Chnpter  XIV. 


2G4 


MODERN  CHROMATICS. 


the  colours  differ  in  luminosity,  analogous  effects  are  ob- 
served. A dull-red  slip  was  placed  on  a vermilion  ground  ; 
the  effect  was  as  though  a quantity  of  grey  had  been  added 
to  the  slip  ; it  looked  more  dingy  and  somewhat  blackish. 
Another  slip  still  darker  and  containing  less  red,  when 
placed  on  the  same  ground,  looked  as  if  it  were  tinged  with 
olive-green  ; a still  darker  slip,  with  still  less  red  colour, 
treated  in  the  same  way,  looked  black  with  a tinge  of  blue  ; 
wlien,  however,  this  last  slip  was  placed  on  a white  ground, 
or  compared  with  true  black,  it  was  seen  that  its  colour 
was  far  from  black.  The  general  result  of  contrasting  col- 
ours  which  differ  much  in  strength,  then,  is  that  the  feebler 
one  appears  either  more  whitish  or  greyish,  or  assumes  the 
complementary  tint  ; the  stronger  one,  on  the  other  hand, 
appears  still  more  intense.  If  the  strong  and  weak  colours 
are  complementary  to  each  other,  then  each  of  them  gains 
in  intensity  and  a])pears  ])urer,  this  gain  seeming  to  be 
greater  in  the  case  of  the  pale  tint.  From  this  it  follows 
that,  while  the  juxtaposition  of  strong  with  feeble  colours 
usually  injures  or  greatly  alters  the  latter,  colours  that  are 
complementary  furnish  an  exception,  the  reason  of  which  is 
evident  at  the  first  glance. 

When  the  pale  or  dark  colours  are  not  complementary  to 
their  more  intense  or  brilliant  rivals,  they  undergo  the  same 
changes  indicated  in  the  table  on  page  245,  the  changes  in  the 
case  of  the  dull  or  pale  colours  being  considerably  greater. 
In  proportion  as  the  colours  are  distant  from  each  other  in 
the  chromatic  circle  do  they  gain  in  saturation  and  beauty  ; 
while  as  they  approach  their  character  is  altered,  and  they 
are  apt  to  look  very  pale,  or,  in  the  case  of  the  dark  col- 
ours, blackish  or  dirty.  This  is  particularly  so  when  the 
brilliant  colour  is  large  in  surface  and  surrounds  the  darker 
one  ; with  reversed  conditions  the  effect  is  not  so  much 
felt.  Thus,  a somewhat  dull  red  near  vermilion  no  longer 
looks  red,  but  brown  ; a dull  orange  tint  on  the  same 
ground  looks  like  a yellowish-brown. 


CONTRAST. 


265 


It  miglit  be  supposed  from  wbat  bas  preceded  that  col- 
ours would  enrich  each  other  only  when  separated  by  a 
large  interval  in  the  chromatic  circle  ; and  from  a purely 
physiological  point  of  view  this  is  indeed  true.  Still,  there 
are  other  influences  of  a more  spiritual  character  at  work, 
which  modify  and  sometimes  even  reverse  this  lower  law. 
Thus,  the  presence  of  paler  colour  in  a painting  near  that 
which  is  richer  often  passes  unperceived,  simply  making  the 
impression  of  a higher  degree  of  illumination.  We  recog- 
nize the  representation  of  a flood  of  light,  and  delight  in  it, 
without  finding  fault  with  the  pale  tints,  if  only  they  are 
laid  with  decision  and  knowledge.  Again,  pale  colour  we 
delight  in  as  representing  the  distance  of  a landscape  ; the 
pale  greenish-greys,  bluish-greys,  and  faint  tints  of  purple 
which  make  it  up,  we  never  think  of  putting  into  envious  com- 
petition with  the  rich  intense  colours  of  the  foreground,  but 
enjoy  each  separately,  and  rejoice  in  the  effect  of  atmosphere 
and  distance,  which  neither  kind  of  colour  alone  by  itself 
could  adequately  render.  That  is  to  say,  for  the  sake  of 
light  and  atmosphere  or  distance,  we  gladly  sacrifice  a large 
portion  of  the  powerful  tints  at  our  disposal,  and  consider 
ourselves  gainers.  The  same  is  also  true  in  another  di- 
rection : we  are  ready  to  make  the  same  sacrifice  for  the 
sake  of  avoiding  monotony  and  gaining  variety,  provided 
only  we  can  justify  the  act  by  a good  reason.  Cases  of 
this  kind  often  occur  in  large  masses  of  foliage,  which,  if 
of  the  same  general  colour,  are  apt  in  a painting  to  look 
monotonous  and  dull,  unless  great  labour  is  bestowed  in 
rendering  the  light  and  shade  and  the  small  differences  of 
tint  which  actually  exist  in  nature.  Under  such  circum- 
stances the  observer  feels  a certain  relief  at  the  presence  of 
a few  groups  of  foliage  which  are  decidedly  paler  in  colour 
than  the  surrounding  masses,  provided  only  there  is  a good 
excuse  for  their  introduction.  Willow-trees  agitated  by 
wind,  and  showing  the  under  sides  of  their  leaves,  which 
are  of  a pale-greenish  hue,  offer  a familiar  example. 


266 


MODERN  CHROMATICS. 


Again,  the  mere  contrast  of  dark  or  dull  tints  enhances  the 
colour  and  luminosity  of  those  that  are  bright,  and  the  ob- 
server receives  the  impression  that  he  is  gazing  at  a mass 
of  gay  and  beautiful  colouring,  scarcely  noticing  the  pres- 
ence of  the  much  larger  quantity  of  tints  that  are  darkened 
by  being  in  deep  shade.  These  darkened  shade-tints  are 
usually  not  variations  of  the  same  hue  as  the  brighter  tints, 
but  are  more  bluish  ; so  that,  technically,  the  combinations 
would  often  present  instances  of  harmful  contrast,  were  it 
not  for  the  fact  that  the  bright  and  dull  tints  do  not  belong 
even  to  the  same  chromatic  circle,  but  to  circles  situated  in 
different  planes,  as  explained  in  the  previous  chapter.  Put- 
ting this  into  more  ordinary  language,  we  should  say  sim- 
ply that  the  strong  contrast  of  light  and  shade  masked  such 
effects  of  harmful  colour-contrast  as  were  present.  There 
is,  however,  another  case  where  we  are  not  so  indifferent  or 
lenient.  AVhcre  two  objects  are  placed  near  each  other  in 
a painting,  and  there  is  good  reason  why  both  should  dis- 
})lay  the  same  colour  with  equal  intensity,  if  one  is  painted 
with  rich  colour,  the  other  with  a pale  or  dark  shade  of  the 
same  colour,  then  the  latter  will  look  either  washed  out  or 
dirty,  and  a bad  elfect  will  be  produced.  As  a familiar 
illustration  of  this  kind  of  effect,  we  may  allude  to  the  use 
in  dress  of  two  widely  differing  shades  of  ribbon,  which 
have  still  the  same  general  colour. 

There  is  a still  more  general  reason  upon  which  the 
jdeasure  that  we  experience  from  contrast  depends.  After 
gazing  at  large  surfaces  filled  with  many  varieties  of  warm 
colour,  skillfully  blended,  we  feel  a peculiar  delight  in 
meeting  a few  mildly  contrasting  tints  ; they  prevent  us 
from  being  cloyed  with  all  the  wealth  of  rich  colouring  so 
lavishly  displayed,  and  their  faint  contradiction  makes  us 
doubly  enjoy  the  richer  portions  of  the  painting.  So  also 
when  the  i)icture  is  mainly  made  up  of  cool,  bluish  tints  : 
it  is  then  extraordinarily  strengthened  and  brightened  by  a 
few  touches  of  warm  colour.  Precisely  the  same  idea 


CONTRAST. 


267 


holds  good  in  drawings  destitute  of  colour,  and  made  up 
merely  of  lines  : if  the  drawing  is  composed  mainly  of  soft 
flowing  curves,  a few  slanting  straight  lines  across  them 
seem  delicious  ; or,  to  take  a parallel  case  from  another 
department  of  art,  in  a discourse  which  is  mainly  grave  in 
tone  the  introduction  of  a few  slight  touches  of  humour 
brightens  and  warms  the  audiepce  most  pleasantly.  If 
there  is  about  an  equal  quantity  of  gay  and  sombre  tints, 
the  effect  is  less  good,  becoming  particularly  bad  when  the 
composition  is  divided  up  into  many  alternate  groups  of 
gay  and  pale  or  sombre  colours,  impartially  distributed. 

Before  concluding  this  chapter,  it  is  necessary  to  add  a 
few  words  with  regard  to  the  simple  contrast  of  light  and 
shade,  that  is,  where  all  the  elements  are  comprised  by 
white,  black,  and  intermediate  shades  of  grey.  As  might 
be  expected  from  what  has  preceded,  when  a light  grey  is 


placed  near  a darker  grey,  the  light  shade  appears  still 
lighter,  the  dark  shade  still  darker.  This  can  be  beautifully 
shown  with  the  aid  of  our  very  convenient  revolving  disks. 
We  take  a black  and  white  disk  painted  as  represented  in 
Fig.  128.  When  this  disk  is  set  in  rotation,  it  will,  on  ac- 
count of  the  mixture  of  the  black  and  white,  produce  a 
series  of  grey  rings,  growing  darker  as  we  proceed  from 


Fig.  128.— Black  and  White  Disk 
for  Experiment  on  Contrast. 


Fig.  129.— Showing?  the  result  when 
disk,  Fig.  12?,  is  set  into  rapid 
rotation. 


268 


MODERN  CHROMATICS. 


the  circumference  toward  the  centre.  Each  ring,  from  the 
circumstances  of  the  case,  will  as  a matter  of  fact  have  an 
absolutely  uniform  shade  of  grey,  but  nevertheless  it  will 
not  appear  so  to  the  observer.  The  rings  will  appear  to  him 
not  uniform,  but  shaded,  the  lightest  shade  being  always 
turned  inward  toward  the  centre,  as  roughly  represented  in 
Fig.  129.  Where  a ring  comes  in  contact  with  one  that  is 
lighter  than  itself,  it  is  made  to  look  darker  ; with  one 
darker  than  itself,  to  look  lighter.  The  same  effect  can  be 
observed  by  painting  a series  of  pieces  of  paper  with  differ- 
ent flat  tints  of  grey,  and  then  arranging  them  to  corre- 
spond with  the  disk  experiment  ; they  will  present  an 
appearance  like  that  indicated  by  Fig.  130.  It  is  hardly 


Fig.  130. — Slips  of  grey  paper  made  to  appear  shaded  by  contrast. 


necessary  to  add  that  in  iight-and-shade  drawings,  as  well 
as  in  nature,  appearances  of  this  kind,  more  or  less  modified, 
are  of  constant  occurrence.  One  of  the  most  common  cases 
is  where  range  after  range  of  mountains  rise  behind  one 
another,  the  lower  portions  of  the  distant  ranges  appearing 
lighter  than  the  upper  outlines.  During  rain,  ranges  of 
hills  often  exhibit  this  phenomenon  with  astonishing  dis- 
tinctness. Even  when  the  light  and  dark  tints  are  at  quite 
a distance  from  each  other,  the  phenomena  of  contrast  pre- 
sent themselves  if  there  is  much  difference  in  the  depth  of 


CONTRAST. 


269 


the  two  sets  of  shades.  It  is  a very  common  experience 
that  the  sky  of  a landscape  in  a drawing  turns  out  too  pale 
after  the  rest  of  the  drawing  is  completed.  Contrasted 
with  the  white  paper  of  the  unfinished  sketch,  it  may  look 
quite  right  ; but,  after  the  deeper  tones  of  the  distance  and 
foreground  are  added,  may  become  quite  insignificant. 
Again,  a few  decidedly  black  touches  in  a drawing  will 
often  by  contrast  lighten  up  portions  that  had  previously 
seemed  considerably  too  dark ; or  a few  touches  of  pure 
white  will  apparently  darken  spaces  that  had  seemed  pale 
and  weak.  In  each  case  the  observer  is  furnished  by  exter- 
nal means  with  a standard  for  measuring  the  depth  of  the 
shade,  and  induced  to  use  it  rather  than  his  memory.  By 
the  skillful  employment  of  contrast,  drawings  in  light  and 
shade  can  be  made  to  appear  luminous  and  brilliant,  or  rich 
and  deep  ; neglect  of  this  element  produces  tameness  and 
feebleness.  The  contrast  of  light  with  dark  shades  is  not 
inferior  in  power  to  that  of  warm  with  cool  tints  ; and,  in 
point  of  fact,  the  contrast  of  white  with  black  is  the  strong- 
est case  of  contrast  possible.  We  have  on  the  one  hand 
the  presence  of  all  the  colours,  on  the  other  their  total  ab- 
sence. Hence,  as  has  been  noticed  before,  the  contrast  that 
takes  place  between  light  and  shade  will  sometimes  mask 
or  even  reverse  that  which  occurs  with  different  colours. 
We  can  perhaps  better  tolerate  a shortcoming  in  the  matter 
of  colour-contrast  than  in  that  of  light  and  shade  ; if  the 
latter  is  right  and  powerful,  we  forgive  a limited  amount  of 
inferiority  in  the  former,  merely  remarking  that  the  work 
is  rather  slight  or  pale  in  colour,  but  not  on  that  account 
pronouncing  a verdict  of  total  condemnation.  On  the  other 
hand,  if  the  colour  as  such  is  right,  but  the  depth  of  the 
different  tints  mostly  defective,  then  the  whole  is  spoiled, 
and  we  contemplate  the  tints,  lovely  enough  in  isolation, 
with  no  satisfaction.  We  forgive,  then,  a partial  denial  of 
the  truths  of  colour  more  easily  than  those  of  light  and 
shade,  wliich  probably  is  a result  of  the  natm*e  of  the  opti- 


270 


MODERN  CHROMATICS. 


cal  education  of  the  race.  For  the  human  race,  thus  far, 
light  and  shade  has  been  the  all-important  element  in  the 
recognition  of  external  objects  ; colour  has  played  only  a 
subordinate  part,  and  has  been  rather  a source  of  pleasure 
than  of  positive  utility. 

All  that  has  been  said  with  regard  to  the  contrasts  of 
white,  black,  and  grey,  with  slight  modification,  applies  to 
any  single  colour  taken  by  itself  ; for  instance,  to  drawings 
executed  in  one  colour  only,  such  as  blue  or  brown.  From 
this  it  results  that  every  colour  is  capable  of  exhibiting  two 
kinds  of  contrast,  viz.,  that  involved  by  competition  with 
other  colours  and  that  of  mere  light  and  shade. 

The  contrast  of  white,  black,  and  grey  with  the  series  of 
positive  colours  remains  to  be  noticed.  Taking  up  these  in 
order,  we  find  that  red  when  placed  on  a white  ground  ap- 
pears darker  and  rather  more  intense  in  hue  ; on  a black 
ground  it  becomes  tinted  somewhat  orange-red,  and  looks 
of  course  more  luminous.  Both  these  effects  are  probably 
due  ultimately  to  mere  contrast  of  light  and  shade  ; the 
white  ground  makes  the  red  by  contrast  look  darker.  But 
we  are  accustomed  to  see  red  when  it  is  darkened  recede 
from  orange  and  approach  pure  red,  or  even  perhaps  to  be- 
come somewhat  pui’plish  ; hence  it  appears  so  in  this  case  ; 
it  is  an  instance  of  expectant  attention.  When  red  is 
placed  on  a black  ground,  it  is  made  by  contrast  to  look 
more  luminous  ; but  we  are  accustomed  to  see  luminous  red 
become  tinted  with  an  orange  hue  ; hence  the  result.  Red 
on  grey  grounds  of  various  depths  undergoes  modifications 
corresponding  to  those  just  mentioned.  Pale  red,  i.  e.,  red 
mixed  with  much  white,  on  a white  ground,  gains  in  inten- 
sity of  colour  ; on  a black  or  dark-grey  ground  it  loses 
intensity,  and  approximates  to  pure  white  in  appearance. 
Here  the  contrast  of  light  and  shade  is  so  strong  as  to  cause 
the  colour  to  pass  almost  unperceived  ; or  we  may  say  pale 
red  really  does  approach  much  nearer  to  pure  white  than 
black,  and  hence  is  at  last  accepted  for  it.  Dark,  dull  red 


CONTRAST. 


271 


on  a white  ground  may  be  mistaken  for  brown  ; on  a black 
ground  it  appears  more  luminous  and  more  red.  Orange 
on  a white  ground  looks  darker  and  more  reddish,  on  a 
black  ground  more  luminous  and  yellow.  The  other  effects 
correspond  with  those  described  in  the  case  of  red.  Yel- 
low on  a white  ground  appears  darker  and  more  greenish 
than  on  a black  ground  ; in  the  latter  case  it  is  particu- 
larly brilliant,  and  the  black  also  looks  well,  taking  on  a 
bluish  tint.  Dark  yellow  on  a white  ground  looks  brown 
or  greenish-brown  ; on  a black  ground  its  colour  is  dis- 
played to  more  advantage.  Pale  yellow  on  a white  ground 
is  apt  to  look  greenish,  on  a black  ground  to  appear  whitish. 
Yellow  and  grey  or  black  constitute  a pleasant  combina- 
tion, of  which  extensive  use  has  been  made  in  nature  and 
art.  Green  on  a white  ground  looks  deeper  and  richer,  on 
a black  ground  somewhat  paler  ; by  contrast  the  black  is 
made  to  look  somewhat  reddish  or  rusty.  Green  causes 
grey  to  appear  reddish  ; the  effect  is  particularly  marked 
when  the  grey  has  about  the  same  luminosity  with  the 
green,  also  when  both  are  in  the  shade.  Cyan-blue  on 
white  appears  darker  and  perhaps  more  greenish  than  on 
black.  Blue  on  white  appears  dark  and  rich,  but  shows  no 
tendency  to  green  ; on  black,  by  contrast,  it  becomes  more 
luminous.  The  same  is  true  wdth  blue  on  grey  ; the  latter 
acquires  a somewhat  yellowish  or  rusty  hue.  The  action 
with  violet  is  similar  to  that  of  blue. 

From  the  foregoing  it  is  evident  that  the  contrast  of 
black,  white,  and  grey  with  the  colours  depends  mainly  on 
an  apparent  increase  or  diminution  of  their  luminosity, 
whereby  in  most  cases  their  apparent  hue  is  affected  owing 
to  association.  In  the  case  of  the  colder  colours,  the  tint 
of  the  grey  or  black  ground  is  affected,  and  shows  a ten- 
dency toward  a hue  complementary  to  the  colour  em- 
ployed. We  associate  grey  with  blueness,  and  where  the 
effect  is  such  as  contradicts  this  habitual  association  it  is 
disagreeable ; on  the  other  hand,  grey  with  yellow  forms 


272 


MODERN  CHROMATICS. 


an  agreeable  contrast,  as  the  yellow  tends  to  make  the  grey 
look  more  bluish,  and  thus  corrects  any  yellowish  or  rusty 
appearance  connected  with  it.  Ibis  claimed  in  some  works 
on  colour  that  the  complementary  tints  furnished  by  the 
pure  grey  react  on  and  strengthen  the  colours  which  call 
them  forth.  An  eye  which  is  tired  by  gazing  at  green  is 
indeed  rested  by  looking  at  its  complement,  i.  e.,  at  a mix- 
ture of  red  and  violet,  and  afterward  will  see  the  green  with 
more  vividness  ; but  it  is  difficult  to  understand  how  the 
presentation  of  red,  violet,  and  green,  or,  what  is  the  same 
thing,  grey  light,  can  materially  refresh  the  eye,  or  restore 
its  temporarily  exhausted  power.  In  the  case  of  pale  tints, 
an  effect  of  this  character  does  indeed  seem  to  take  place, 
but  we  must  attribute  it  rather  to  an  act  of  judgment  than 
to  a physiological  cause. 


CHAPTER  XYl. 


ON  THE  SMALL  INTERVAL  AND  ON  GRADATION. 

In  the  preceding  chapter  we  have  seen  that,  when  two 
colours  which  are  nearly  identical  are  contrasted,  each  is 
made  to  appear  less  intense  or  saturated  : red  with  orange- 
red,  yellow  with  orange-yellow,  cyan-hlue  with  blue,  are 
examples  of  such  combinations.  From  this  it  might  be 
supposed  that,  in  chromatic  compositions,  it  would  not  be 
allowable  to  place  colours  thus  nearly  related  in  close  juxta- 
position. It  is,  however,  found  in  practice  that  colours 
which  are  distant  from  each  other  in  the  chromatic  circle 
by  a small  interval  can  be  associated  without  detriment 
under  certain  conditions.  If  the  two  colours  express  a varia- 
tion in  the  luminosity  of  one  and  the  same  coloured  surface, 
they  do  not  come  into  hurtful  competition,  and  we  receive 
the  impression  of  a single  coloured  surface,  more  highly 
illuminated  in  certain  portions.  The  scarlet  coat  of  a sol- 
dier when  shaded  appears  red  ; the  sunlit  portion  is  orange- 
red.  Grass  in  the  sunshine  acquires  a yellowish-green  hue  ; 
in  the  shade  its  colour  is  more  bluish.  But  neither  of 
these  cases  produces  on  us  a disagreeable  effect,  for  we  re- 
gard them  as  the  natural  consequences  of  the  kind  of  illu- 
mination to  which  these  objects  are  exposed.  The  effect  is 
not  disagreeable  even  in  mere  ornamental  painting,  if  it  is 
seen  that  the  two  tints  are  intended  to  express  different 
degrees  of  luminosity  of  the  same  constituent  of  the  design, 
even  though  this  be  only  arabesque  tracery.  From  this 
explanation  it  follows  that  the  two  contiguous  tints  should 


274 


MODERN  CHROMATICS. 


have  their  luminosities  arranged  so  as  to  correspond  to 
nature  ; otherwise  a contradictory  effect  would  he  produced. 
The  following  table  gives  a series  of  small  intervals,  ar- 
ranged properly  as  to  luminosity;  and  it  will  be  seen  to  cor- 
respond to  the  relative  luminosities  of  the  colours  of  the 
spectrum,  or  of  the  colours  which  taken  together  make  up 
white  light  (see  Chapter  III.)  : 


Taule  of  Small  Intervals. 


Darker. 

Red 

Orange-red 

Orange 

Orange-yellow.  . . 
Yellowish-green  . 

Green 

Cyan-blue 

Rlue 

Ultramarlne-blue 

Violet 

Purple 


Lighter, 

Orange-red. 

Orange. 

Orange-yellow. 

Yellow. 

Greenish-yellow. 

Yellowish-green. 

Green. 

Cyan-blue. 

Blue. 

Purple. 

Red. 


It  will  be  noticed  that  the  colours  under  the  heading  “ Dark- 
er ” are  really  the  shade-tints  of  the  series  opposite  them, 
and  the  difference  may  often  be  greater  tlian  that  indicated 
in  the  table.  One  of  the  commonest  of  these  intervals  is 
that  of  yellow  deepening  into  orange-yellow.  In  sunsets 
yellow  scarcely  occurs  without  undergoing  a change  of  this 
kind  ; it  is  almost  the  rule  with  yellow  flowers  ; and  even 
the  pale,  broken,  subdued  yellowish-browns  of  many  natu- 
ral objects  manifest  the  same  tendency.  The  relations  of 
greenish-yellow,  etc.,  to  green  are  shown  beautifully  by 
foliage  under  sunlight,  while  the  interval  of  cyan-blue  to 
blue  or  to  ultramarine-blue  is  displayed  on  the  grandest 
scale  by  the  sky.  In  brilliant  sunsets  the  first  and  last  pair 
of  intervals  are  of  constant  occurrence  ; in  fact,  we  can 
scarcely  think  of  a sunset  without  calling  up  in  imagination 


THE  SMALL  INTERVAL  AND  GRADATION. 


275 


red  and  purple.  The  interval  greenish-yellow  and  yellow 
is  not  included  in  the  list ; it  is  perhaps  less  easy  to  tolerate 
than  any  of  the  others  ; we  like  to  see  yellow  luminous  or 
rich,  that  is,  passing  into  orange  ; but,  when  it  begins  to 
become  decidedly  greenish,  we  hesitate,  unless  there  is 
some  good  reason  for  accepting  it.  When  the  small  inter- 
val is  used,  and  the  two  tints  are  put  more  or  less  into  com- 
petition by  belonging  to  different  surfaces,  the  effect  is  less 
good,  unless  it  is  accounted  for  by  the  nature  of  the  illumi- 
nation, or  in  some  other  equally  satisfactory  way. 

Much  of  the  above  applies  to  the  case  where  the  colours 
pass  into  each  other  by  gentle  and  insensible  gradations,  so 
that  the  observer  is  quite  at  a loss  to  say  where  one  ends 
and  the  other  begins.  Here,  as  before,  colours  which  are 
nearly  related,  or  separated  only  by  a small  interval,  blend 
harmoniously  into  each  other  and  produce  a good  effect. 
The  reason  of  this  is  again  found  mainly  in  our  precon- 
ceived ideas  of  the  changes  which  coloured  surfaces  under- 
go when  more  or  less  strongly  illuminated.  If  the  colours 
are  quite  distant  from  each  other  in  the  chromatic  circle,  a 
rapid  transition  from  one  to  the  other,  by  blending,  pro- 
duces always  a strange  and  often  a disagreeable  effect.  A 
yellow  surface  distinctly  opposed  or  contrasted  to  a blue 
surface  often  gives  a good  effect ; but,  if  it  passes  by  a 
series  of  quick  gradations  into  blue,  the  effect  is  b'ad  ; it  is 
as  though  it  tried  to  assert  at  the  same  time  that  it  was 
warm  and  luminous  as  well  as  cold  and  dark.  In  the  case 
of  the  sky,  it  is  true  that  we  have,  toward  sunset,  the  yel- 
low portions  blending  below  into  orange  and  red,  and  above, 
by  a long  and  slow  series  of  gradations,  into  blue  ; but  the 
distance  between  the  blue  and  yellow  is  large,  and  they  are 
separated  by  a series  of  neutral  tints,  and  we  think  of  the 
whole  as  an  effect  produced  by  apparent  nearness  or  dis- 
tance from  the  sun.  Even  in  a case  like  this,  many  artists 
prefer  not  to  include  in  their  paintings  too  much  of  the 
upper  blue,  and  thus  are  able  to  give  more  decided  expres- 


276 


MODERN  CHROMATICS. 


sion  to  the  warmth  and  brightness  of  the  sky.  When  we 
see  in  nature  a field  of  grass  gradually  growing  decidedly 
red,  we  think  of  clover  as  the  excuse,  without,  nevertheless, 
being  particularly  edified  by  its  presence.  In  some  moun- 
tain lakes,  such  as  the  Konigssee,  we  find  the  blue-green 
water  actually  passing  in  some  places  by  rather  quick  gra- 
dations into  a purplish-red.  The  rapid  transition  into  this 
almost  complementary  hue  produces  an  effect  which  seems 
strange  and  almost  incredible  to  those  who  for  the  first  time 
behold  it.  When  the  cause  is  recognized,  we  learn  to  look 
upon  the  purple  patches  as  marking  the  shallower  water, 
and,  having  accepted  the  effect  as  reasonable,  we  soon  find 
ourselves  enchanted  by  it,  and  always  remember  it  for  its 
strange  beauty. 

When  two  colours  differing  considerably,  not  only  in 
hue  but  in  saturation,  or  simply  in  the  latter  respect,  blend 
rapidly  into  each  other  on  the  same  surface,  we  always- 
require  a reason  for  the  change  of  tint ; and,  if  none  is  fur- 
nished, the  effect  is  apt  to  appear  absurd,  and  resemble 
somewhat  the  case  of  a man  who  at  one  moment  is  calm 
and  cool,  and  the  next,  without  obvious  reason,  tender  and 
pathetic.  When  we  find  the  cool  grey  or  greyish-brown 
tones  on  the  surface  of  a cliff  suddenly  becoming  rose- 
tinted,  we  require  an  explanation  of  the  change,  and  are 
quite  satisfied  if  told  that  the  top  of  the  cliff  is  still  illumi- 
nated by  the  sinking  sun  ; if,  however,  it  is  midday,  we  are 
forced  to  think  of  red  veins  of  some  foreign  substance  dis- 
seminated through  the  sober  rock,  and  wonder  what  it  can 
possibly  be,  and  wish  it  were  away.  All  this  forms  One  of 
the  minor  reasons  why  painters  like  to  keep  their  tints  to- 
gether in  large  masses,  the  bright  warm  colours  in  one 
place,  the  cool  pale  tints  in  another. 

One  of  the  most  important  characteristics  of  colour  in 
nature  is  the  endless,  almost  infinite  gradations  which  al- 
ways accompany  it.  It  is  impossible  to  escape  from  the 
delicate  changes  which  the  colour  of  all  natural  objects  un- 


THE  SMALL  INTERYAL  AND  GRADATION. 


277 


dergoes,  owing  to  the  way  the  light  strikes  them,  without 
taking  all  the  precautions  necessary  for  an  experiment  in  a 
physical  laboratory.  Even  if  the  surface  employed  be 
white  and  flat,  still  some  portions  of  it  are  sure  to  be  more 
highly  illuminated  than  others,  and  hence  to  appear  a little 
more  yellowish  or  less  greyish  ; and,  besides  this  source  of 
change,  it  is  receiving  coloured  light  from  all  coloured  ob- 
jects near  it,  and  reflecting  it  variously  from  its  different 
portions.  If  a painter  represents  a sheet  of  paper  in  a pic- 
ture by  a uniform  white  or  grey  patch,  it  will  seem  quite 
wrong,  and  can  not  be  made  to  look  right  till  it  is  covered 
by  delicate  gradations  of  light  and  shade  and  colour.  We 
are  in  the  habit  of  thinking  of  a sheet  of  paper  as  being 
quite  uniform  in  tint,  and  yet  instantly  reject  as  insufficient 
such  a representation  of  it.  In  this  matter  our  unconscious 
education  is  enormously  in  advance  of  our  conscious  ; our 
memory  of  sensations  is  immense,  our  recollections  of  the 
causes  that  produce  them  utterly  insigniflcant ; and  we  do 
not  remember  the  causes  mainly  because  we  never  knew 
them.  It  is  one  of  the  tasks  of  the  artist  to  ascertain  the 
causes  that  give  rise  to  the  highly  complex  sensations  which 
he  experiences,  even  in  so  simple  a case  as  that  just  consid- 
ered. From  this  it  follows  that  his  knowledge  of  the  ele- 
ments that  go  to  make  up  chromatic  sensations  is  very  vast 
compared  with  that  of  ordinary  persons  ; on  the  other 
hand,  his  recollection  of  mere  chromatic  sensations  may  or 
may  not  be  more  extensive  than  theirs.  Hence  it  follows 
that  it  requires  long  training  to’ acquire  the  power  of  con- 
sciously tracing  fainter  gradations  of  colour,  though  much 
of  the  pleasure  experienced  by  their  passive  reception  can 
be  enjoyed  without  previous  labour. 

These  ever-present  gentle  changes  of  colour  in  all  natu- 
ral objects  give  to  the  mind  a sense  of  the  richness  and 
vastness  of  the  resources  of  Nature  ; there  is  always  some- 
thing more  to  see,  some  new  evanescent  series  of  delicate 
tints  to  trace  ; and,  even  where  there  is  no  conscious  study 


278 


MODERN  CHROMATICS. 


of  colour,  it  still  produces  its  effect  on  the  mind  of  the  be- 
holder, giving  him  a sense  of  the  fullness  of  Nature,  and  a 
dim  perception  of  the  infinite  series  of  gentle  changes  by 
which  she  constantly  varies  the  aspects  of  the  commonest 
objects.  This  orderly  succession  of  tints,  gently  blending 
into  one  another,  is  one  of  the  greatest  sources  of  beauty 
that  we  are  acquainted  with,  and  the  best  artists  constantly 
strive  to  introduce  more  and  more  of  this  element  into  their 
works,  relying  for  their  triumphs  far  more  on  gradation 
than  on  contrast.  The  greatest  effects  in  oratory  are  also 
produced  by  corresponding  means  ; it  is  the  modulation  of 
the  tone  and  thought,  far  more  than  sharp  contrasts,  that  is 
effective  in  deeply  moving  audiences.  We  are  very  sensi- 
tive to  the  matter  of  modulation  even  in  ordinary  speech, 
and  instantly  form  a general  judgment  with  regard  to  the 
degree  of  cultivation  and  refinement  of  a stranger  from  the 
mode  in  which  a few  words  are  pronounced.  All  this  has 
its  parallel  in  the  use  of  colour,  not  only  in  painting,  but 
also  in  decoration.  Ruskin,  speaking  of  gradation  of  col- 
our, says  : “ You  will  find  in  practice  that  brilliancy  of  hue 
and  vigor  of  light,  and  even  the  aspect  of  transparency  in 
shade,  are  essentially  dependent  on  this  character  alone  ; 
hardness,  coldness,  and  opacity  resulting  far  more  from 
equality  of  colour  than  from  nature  of  colour.”  In  another 
place  the  same  author,  in  giving  advice  to  a beginner,  says  : 
“ And  it  does  not  matter  how  small  the  touch  of  colour  may 
be,  though  not  larger  than  the  smallest  pin’s  head,  if  one 
part  of  it  is  not  darker  than  the  rest,  it  is  a bad  touch  ; for 
it  is  not  merely  because  the  natural  fact  is  so  that  your  col- 
our should  be  gradated  ; the  preciousness  and  pleasantness 
of  colour  depends  more  on  this  than  on  any  other  of  its 
qualities,  for  gradation  is  to  colours  just  what  curvature  is 
to  lines,  both  being  felt  to  be  beautiful  by  the  pure  instinct 
of  every  human  mind,  and  both,  considered  as  types,  ex- 
pressing the  law  of  gradual  change  and  progress  in  the 
human  soul  itself.  What  the  difference  is  in  mere  beauty 


THE  SMALL  INTERVAL  AND  GRADATION. 


279 


between  a gradated  and  ungradated  colour  may  be  seen 
easily  by  laying  an  even  tint  of  rose-colour  on  paper  and 
putting  a rose-leaf  beside  it.  The  victorious  beauty  of  the 
rose  as  compared  with  other  flowers  depends  wholly  on  the 
delicacy  and  quantity  of  its  colour-gradations,  all  other 
flowers  being  either  less  rich  in  gradation,  not  having  so 
many  folds  of  leaf,  or  less  tender,  being  patched  and  veined 
instead  of  flushed.”  * 

All  the  great  colourists  have  been  deeply  permeated  by 
a sentiment  of  this  kind,  and  their  works,  when  viewed 
from  the  intended  distance,  are  tremulous  with  changing 
tints — with  tints  that  literally  seem  to  change  under  the 
eye,  so  that  it  is  often  impossible  for  the  copyist  to  say 
exactly  what  they  are,  his  mixtures  never  seeming  to  be 
quite  right,  alter  them  as  he  will.  Among  modern  land- 
scape paintings,  those  of  Turner  are  famous  for  their  end- 
less quantity  of  gradation,  and  the  same  is  true  even  of  his 
water-colour  drawings.  The  perfect  blending  of  colours, 
for  example,  in  the  sky,  or  in  our  best  representations  of  it, 
produces  an  effect  of  wonderful  softness  and  beauty,  the 
tints  melting  into  each  other  with  a liquid  smoothness  for 
which  we  can  find  no  other  parallel.  The  absolutely  per- 
fect gradation  and  softness  of  the  sky  well  expresses  its 
qualities  as  a gas,  impalpable,  evanescent,  boundless. 

There  is,  however,  another  lower  degree  of  gradation 
which  has  a peculiar  charm  of  its  own,  and  is  very  precious 
in  art  and  nature.  The  effect  referred  to  takes  place  when 
different  colours  are  placed  side  by  side  in  lines  or  dots, 
and  then  viewed  at  such  a distance  that  the  blending  is 
more  or  less  accomplished  by  the  eye  of  the  beholder. 
Under  these  circumstances  the  tints  mix  on  the  retina,  and 
produce  new  colours,  whicli  are  identical  with  tliose  that 

*“  Elements  of  Drawing,”  by  J.  Ruskin.  The  distinguished  artist 
Samuel  Colman  once  remarked  to  the  wi  iter,  that  this  book  not  only  con- 
tained more  that  was  useful  to  the  student  of  art  than  any  previous  work, 
but  that  it  contained  more  than  all  of  them  put  together. 


280 


MODERN  CHROMATICS. 


are  obtained  by  the  method  of  revolving  disks.  (See 
Chapter  X.)  If  the  coloured  lines  or  dots  are  quite  distant 
from  the  eye,  the  mixture  is  of  course  perfect,  and  presents 
nothing  remarkable  in  its  appearance  ; but  before  this  dis- 
tance is  reached  there  is  a stage  in  which  the  colours  are 
blended,  though  somewhat  imperfectly,  so  that  the  surface 
seems  to  flicker  or  glimmer — an  effect  that  no  doubt  arises 
from  a faint  perception  from  time  to  time  of  its  constitu- 
ents. This  communicates  a soft  and  peculiar  brilliancy  to 
the  surface,  and  gives  it  a certain  appearance  of  transpar- 
ency ; we  seem  to  see  into  it  and  below  it.  Dove’s  theory 
of  lustre  has  perhaps  some  bearing  on  this  well-known  phe- 
nomenon. -According  to  Dove,  when  two  masses  of  light 
simultaneously  act  on  the  eyes,  lustre  is  perceived,  provided 
we  are  in  any  way  made  conscious  that  there  are  actually 
txco  masses  of  light.  On  a polished  varnished  table  we  see 
the  surface  by  means  of  its  imperfections,  scratches,  dust, 
etc.,  and  tlien  besides  have  presented  to  us  another  mass  of 
light  which  is  regularly  reflected  from  the  surface  ; the 
table  looks  to  us  lustrous.  The  author,  and  afterward  Dove 
in  a different  way,  succeeded  in  producing  this  lustrous  ap- 
pearance when  only  a single  eye  was  employed,  that  is, 
without  the  aid  of  binocular  vision.*  In  the  case  before 
us,  the  images  of  the  colour-dots  are  more  or  less  superim- 
posed on  tlie  retina,  and  consequently  seen  one  through  the 
other ; and  at  the  right  distance  there  is  some  perception 
of  a lack  of  uniformity,  the  degree  of  blending  varying 
from  time  to  time.  According  to  Dove’s  theory,  we  have 
liere  the  conditions  necessary  for  the  production  of  more  or 
less  soft  brilliancy.  With  bright  complementary  colours 
the  maximum  degree  of  lustre  is  obtained  ; when  the  col- 
ours are  near  each  other  in  the  chromatic  circle,  or  dull  or 
pale,  the  effect  is  not  marked,  but  exists  to  the  extent  of 
making  the  surface  appear  somewhat  transparent.  When 
the  two  colours  are  replaced  simply  by  black  and  white, 
* “ American  Journal  of  Science  and  Arts,”  May,  186-1. 


THE  SMALL  INTEKVAL  AND  GRADATION. 


281 


the  same  lustrous  appearance  is  still  produced.  Sir  David 
Brewster  has  described  an  experiment  which  has  some  bear- 
ing on  these  matters.  If  a wall-paper  is  selected  with  a 
pattern  which  repeats  itself  at  intervals  of  a few  inches,  it 
is  possible,  after  some  practice,  so  to  arrange  the  eyes  as  to 
cause  the  adjacent  and  corresponding  portions  to  seem  to 
coalesce  and  form  a new  picture,  Tyhich  will  in  most  respects 
be  identical  with  that  obtained  by  ordinary  vision.  This 
new  picture  will  not  seem  to  be  at  the  same  distance  from 
the  eye  as  the  real  objects,  and  will  move  with  each  slight 
motion  of  the  head  ; but  what  concerns  us  more  is,  that  it  has 
a certain  appearance  of  transparency  and  beauty  not  found 
in  the  original.  In  this  experiment  two  slightly  dissimilar 
masses  of  light  are  presented  to  the  two  eyes,  and  the  result  is 
an  appearance  of  transparency,  using  this  word  in  its  artis- 
tic sense. 

But  to  return  : the  result  of  this  imperfect  blending  of 
colours  or  of  black  and  white  by  the  eye  is  to  communicate 
to  the  surface  an  appearance  of  clearness,  and  to  remove 
any  idea  of  hardness  or  chalkiness  ; it  is  so  familiar  to  us 
that  we  accept  it  as  quite  natural,  and  only  become  con- 
scious of  its  charm  when  it  is  withdrawn.  As  an  example 
in  nature,  we  have  the  somewhat  distant  sea  under  a bright- 
blue  sky  : the  waves  will  be  mainly  green,  the  spaces  be- 
tween them  blue  ; these  colours  then  blend  into  a sparkling 
greenish-blue,  which  can  not  be  imitated  with  a simple 
mixed  pigment.  Also  in  grasses  viewed  at  some  distance, 
the  yellowish-green,  bluish-green,  reddish,  purplish,  and 
brown  tints,  and  the  glancing  lights,  blend  more  or  less  to- 
gether, and  produce  an  effect  which  can  not  be  reproduced 
by  a single  sweep  of  the  brush.  The  more  distant  foliage 
of  trees  on  hillsides  shows  something  of  this  kind  ; and  it 
does  not  appear  to  be  entirely  absent  even  from  the  dust  on 
a traveled  road,  the  minute  sparkling  grains  of  sand  still 
producing  some  action  on  the  eye  after  they  can  no  longer 
be  distinguished  individually. 


282 


MODERN  CHROMATICS. 


In  fresco-painting,  and  in  scene-painting  for  the  theatre, 
most  extensive  use  is  made  of  this  principle  : at  the  right 
distance  adjacent  tints  blend,  and  what  near  at  hand  seemed 
a mass  of  purposeless  daubs  becomes  an  effective  picture. 
This  same  method  of  mixing  colours  on  the  retina  of  the 
observer  is  also  used  more  or  less  in  oil  painting  with  ex- 
cellent effect  ; it  lends  to  them  a magical  charm,  the  tints 
seeming  purer  and  more  varying  ; the  very  fact  that  the 
appearance  of  the  painting  changes  somewhat  according  as 
the  observer  advances  or  retires  from  it  being  an  advantage, 
communicating  to  it,  as  we  might  say,  a certain  kind  of  life. 
Oil  paintings  in  which  tliis  principle  is  not  employed  labour 
under  one  quite  demonstrable  disadvantage  : as  the  observer 
retires  adjacent  tints  blend,  whether  it  was  the  intention  of 
the  artist  or  not  ; and  if  this  has  not  been  calculated  for,  a 
new  and  inferior  effect  is  pretty  sure  to  be  produced.  In 
Avater-colour  drawings  the  same  mode  of  working  is  con- 
stantly employed  under  the  form  of  stippling,  more  or  less 
formal  ; and  with  its  aid  certain  results  of  transparency 
and  richness  can  be  attained,  which  otherwise  would  be  out 
of  the  reach  of  the  artist.  If  the  stippling  is  formal  and 
quite  evident,  it  is  apt  to  give  a mechanical  look  to  a draw- 
ing, which  is  not  particularly  pleasant ; but  properly  used, 
it  has  great  value,  and  readily  lends  itself  to  the  expression 
of  form.  To  descend  several  steps  lower,  we  find  the  de- 
signers of  wall-papers  and  carpets  employing  this  mode  of 
mixing  colours  and  producing  their  gradations.  In  cash- 
mere  shawls  the  same  principle  is  developed  and  pushed  to 
a great  extent,  and  much  of  their  beauty  is  dependent  on  it. 
Finally,  in  etchings,  engravings,  and  pen-and-ink  drawings, 
we  have  other  examples  of  its  application  ; their  clearness, 
transparency,  and  sparkling  effect  being  mainly  due  to  the 
somewhat  imperfect  blending  of  the  black  and  white  lines. 
This  effect  can  best  be  perceived  by  comparing  them  with 
lithographs,  or,  better  still,  with  Indian  ink  or  sepia  drawings. 
The  sky-like  softness  of  the  last  two  is  very  lovely,  and,  in 


THE  SMALL  INTERVAL  AND  GRADATION. 


283 


some  respects,  very  true  to  nature  ; but  if  in  them  the  line 
manner  is  entirely  avoided,  they  are  a little  apt  to  show  a 
lack  of  transparency  in  the  deeper  masses  of  shade,  and  to 
look  heavy  or  dull.  This  result  is  avoided  by  purposely 
introducing  a certain  amount  of  line-drawing,  either  with 
the  pen  or  a small  brush.  We  have  in  nature  a great 
variety  of  appearances,  and  the  various  methods  of  art  are 
calculated  to  represent  one  or  the  other  of  them  more  or 
less  perfectly  ; but  there  is  no  single  kind  of  art  manipula- 
tion which  will  deal  equally  well  with  all. 

Finally,  it  is  to  be  remarked  that  when  colour  is  used 
simply  for  ornamental  purposes,  blending  or  gradation 
becomes  of  subordinate  importance.  This  is  the  case,  for 
exmaple,  where  the  design  is  worked  out  solely  in  flat  tints. 
Work  of  this  kind,  where  the  fancy  is  not  allowed  to  inter- 
fere much  with  the  general  correctness  of  the  drawing  or 
colour,  forms  one  of  the  first  steps  by  which  painting  gradu- 
ally passes  over  into  pure  ornamental  design.  We  have 
here  colours  arranged  in  harmonious  masses,  bounded  by 
sharp  outlines,  often  definitely  traced  in  black,  and  are 
pleased  with  them,  and  with  the  beautiful,  correct  outlines. 
All  gradation  and  blending  of  colours  is  abolished,  and  this 
fact  alone  announces  to  us,  in  an  emphatic  way,  that  the 
design  makes  no  pretension  to  realistic  representation  ; we 
are  pleased  with  the  colours  and  outlines,  and  are  rather 
surprised  to  find  how  much  can  be  accomplished  by  them  ; 
and  if  gold  is  introduced  in  the  background  or  draperies, 
its  presence  only  adds  to  the  general  effect.  By  insensible 
changes  the  figures  of  men  and  animals,  etc.,  become  more 
conventional  or  grotesque,  as  in  heraldry,  until  finally  there 
is  no  attempt  made  to  portray  any  particular  natural  object. 
Sygffestions  are  taken  from  objects  in  nature,  Avliich  are 
used  in  much  the  same  way  as  by  musical  composers.  The 
intention,  however,  is  the  production  of  a beautiful  design 
which  shall  serve  to  ornament  something  else,  as  a woven 
stuff,  a vase,  or  the  wall  of  a building.  As  gradation  is 


284 


MODERN  CHROMATICS. 


one  of  our  most  efficient  modes  of  giving  vrork  in  colour  a 
thoroughly  realistic  appearance,  it  evidently  can  not  he 
much  employed  in  ornament,  where  it  is  an  object  to  avoid 
any  imputation  of  intentional  realism. 

In  closing  this  chapter  it  may  be  well  to  allude  to  a 
singular  effect  often  produced  by  insensible  gradation  on 
natural  objects,  or  on  their  representations  in  paintings. 
W e have  seen  that  a coloured  surface  having  a well-defined 
shape,  when  placed  on  a grey  ground,  is  capable  through 
contrast  of  causing  the  ground  to  appear  of  the  comple- 
mentary colour.  For  example,  a grey  square  or  a green 
ground  will  appear  as  though  tinted  with  rose-colour.  If, 
however,  the  green  passes  into  the  grey  by  insensible  gra- 
dations, the  matter  may  be  so  arranged  that  a small  amount 
of  green  causes  the  Avhole  surface  to  appear  green,  when 
most  of  it  really  is  grey.  This  effect  is  often  seen  on  ]*ocks 
partially  covered  with  green  moss  : a few  small  patches  on 
the  side  exposed  to  the  light  will  have  a bright-green  hue  ; 
some  of  the  surface  in  the  shade  will  be  tinted  dark  green, 
this  colour  passing  gently  into  brown  or  grey,  with  here 
and  there  a few  quite  small  touches  of  olive-green.  Three 
fourths  of  the  surface  of  the  shaded  side  of  the  rock  will 
then  be  really  grey  or  brown,  but  nevertheless  the  whole 
will  appear  to  be  dark  green.  Another  very  common 
example  is  furnished  by  the  foliage  of  trees  standing  so 
that  the  sun  appears  to  be  over  them.  Under  these  circum- 
stances their  tops  and  sides  catch  the  sunbeams  and  appear 
of  a bright  yellowish-green  ; the  rest  of  the  tree  is  in  the 
shade,  and  appears  at  first  sight  of  a darker  green,  and  is 
always  so  painted  by  beginners.  If,  however,  the  colour  is 
examined  through  an  aperture  about  the  size  of  a pea,  cut 
in  a piece  of  white  cardboard,  it  will  be  found  that  the  real 
colour  is  a somewhat  greenish  grey.  On  retiring  farther 
from  the  tree,  this  colour  of  its  shady  side  will  often  change 
to  a pure  grey,  yet  to  a casual  observer  it  will  still  appear 
green.  Quite  wonderful  effects  have  been  obtained  by 


THE  SMALL  INTERVAL  AND  GRADATION. 


285 


some  artists  from  the  recognition  of  the  principle  here 
involved  ; their  calm  resignation  of  every  trace  of  local 
colouring,  and  acceptance  in  its  place  of  some  kind  of  grey, 
imparting  to  their  pictures  a high  degree  of  aerial  perspec- 
tive and  of  apparent  luminosity. 


CHAPTER  XVII. 


ON  THE  COMBINATION  OF  COLOURS  IN  PAIRS  AND 
TRIADS. 

In  the  previous  portions  of  this  work  we  have  dealt 
with  facts  that  are  capable  of  more  or  less  rigorous  demon- 
stration ; but  we  now  encounter  a great  series  of  problems 
that  can  not  be  solved  by  the  methods  of  the  laboratory,  or 
by  the  aid  of  a strictly  logical  iwocess.  Why  a certain 
combination  of  colours  pleases  us,  or  why  we  are  left  cold 
or  even  somewhat  shocked  by  another  arrangement,  are 
questions  for  which  we  can  not  always  frame  answers  that 
are  satisfactory  even  to  ourselves.  There  is  no  doubt  that 
helpful  and  harmful  contrast  have  a very  great  influence  on 
our  decision,  as  will  hereafter  be  pointed  out  ; but  besides 
this,  we  are  sometimes  influenced  by  obscure  and  even 
unknown  considerations.  Among  these  may  perhaps  be 
found  inherited  tendencies  to  like  or  dislike  certain  com- 
binations or  even  colours  ; influence  of  the  general  colour- 
atmosphere  by  which  we  are  surrounded ; training  ; and 
also  a more  or  less  delicate  nervous  susceptibility. 

The  author  gives  below,  in  the  form  of  tables,  some  of 
the  results  furnished  by  experience,  and  takes  pleasure  in 
acknowledging  his  indebtedness  to  Briicke  and  to  Chevreul 
for  much  of  the  information  contained  in  them. 

Spectral  red  * with  blue  gives  its  best  combination. 

Spectral  red  with  green  gives  a strong  but  rather  hard  combination. 


* A red  between  carmine  and  vermilion. 


COMBINATION  OF  COLOURS  IN  PAIRS  AND  TRIADS.  287 


Spectral  red  with  yellow  gives  an  inferior  combination. 

Spectral  red  with  red  lead  gives  a bad  combination. 

Spectral  red  with  violet  gives  a bad  combination. 

If  gold  be  substituted  for  the  yellow  pigment,  the  com- 
bination becomes  excellent.  Red  and  yellow  also  make  a 
better  combination  when  the  red  inclines  to  purple  and  the 
yellow  to  greenish-yellow.  The  combination  red  and  yellow 
is  also  improved  by  darkening  the  yellow  or  both  colours  ; 
this  causes  the  yellow  to  appear  like  a soft  olive-green  (R.). 
The  combination  red  and  green  is  also  improved  by  darken- 
ing both  colours,  or  the  green  alone  (R.). 

Vermilion  with  blue  gives  an  excellent  combination. 

Vermilion  with  cyan-blue  gives  an  excellent  combination. 

Vermilion  with  green  gives  an  inferior  combination. 

Vermilion  wdth  yellow  gives  an  inferior  combination. 

Vermilion  with  violet  gives  a bad  combiuation. 

Vermilion  and  gold  furnish  an  excellent  combination. 
The  combination  vermilion  and  yellow  is  improved  some- 
what by  darkening  the  yellow  ; if  it  is  considerably  dark- 
ened, it  tells  as  a soft  olive-green  (R.).  Vermilion  and 
green  are  better  when  the  green  or  both  colours  are  much 
darkened  (R.). 

Red  lead  with  blue  gives  an  excellent  combination. 

Red  lead  with  cyan-blue  gives  an  excellent  combination. 

Red  lead  with  blue-green  gives  a strong  but  disagreeable  combination. 

Red  lead  with  yellowish-green  gives  a tolerably  good  combination. 

Red  lead  with  yellow  gives  quite  a good  combination. 

Red  lead  wdth  orange  gives  quite  a good  combination. 

The  combination  red  lead  and  bluish-green  is  improved 
by  darkening  the  green  or  both  the  colours  (R.).  Red  lead 
gives  a better  combination  with  a yellow  having  a corre- 
sponding intensity  or  saturation  ; if  the  yellow  is  too  bright, 
the  effect  is  inferior  (R.).  The  combination  red  lead  and 

yellow  is  much  better  than  red  and  orange.  The  last  two 
13 


288 


MODERN  CHROMATICS. 


combinations  given  in  the  table  are  of  course  cases  where 
the  small  interval  is  employed.  (See  Chapter  XYL) 

Orange  with  cyan-blue  gives  a good  and  strong  combination. 

Orange  with  ultramarine  gives  a good  and  strong  combination. 

Orange  with  green  gives  a good  combination. 

Orange  with  violet  gives  a moderately  good  combination. 

Orange-yellow  with  ultramarine  gives  its  best  combination. 

Orange-yellow  with  cyan-blue  gives  not  quite  so  good  a combination. 

Orange-yellow  with  violet  gives  a good  combination. 

Orange-yellow  with  purple  gives  a good  combination. 

Orange-yellow  with  purple-red  gives  an  inferior  combination. 

Orange-yellow  with  spectral  red  gives  an  inferior  combination. 

Orange-yellow  with  sea-green  gives  a bad  combination. 

Yellow  with  violet  gives  its  best  combinations. 

Yellow  with  purple-red  gives  good  combinations. 

Yellow  with  purple  gives  good  combinations. 

Yellow  with  spectral  red  gives  inferior  combinations. 

Yellow  with  blue,  inferior  to  orange-yellow  and  blue. 

Yellow  with  blue-green  gives  one  of  the  worst  possible  combinations. 

Yellow  with  green  gives  bad  combinations. 

The  comhination  yellow  and  spectral  red  is  improved  by 
darkening  the  yellow  (IT).  l>lue-green  and  yellow,  both 
much  darkened,  give  a better  combination  (R.).  According 
to  Chevreul,  yellow  gives  with  green  a good  and  lively 
combination  ; to  this  the  author  can  not  agree,  although  it 
is  true  that  the  effect  is  improved  by  darkening  the  yellow 
considerably.  Chrome-yellow  and  emerald-green  give  com- 
binations that  are  not  bad  when  both  the  colours  are  very 
much  darkened  (R.). 

Greenish-yellow  with  violet  gives  its  best  combinations. 

Greenish-yellow  with  purple  gives  good  combinations. 

Greenish-yellow  with  purplish-red  gives  good  combinations. 

Greenish-yellow  with  vermilion  gives  strong  but  hard  combinations. 

Greenish-yellow  with  spectral  red  gives  strong  but  hard  combinations 

Greenish-yellow  with  red  lead  gives  tolerably  good  combinations. 


COMBINATION  OF  COLOURS  IN  PAIRS  AND  TRIADS.  289 


Greenish-yellow  with  orange-yellow  gives  bad  combinations. 

Greenish-yellow  with  cyan-blue  gives  bad  combinations. 

Greenish-yellow  with  ultramarine  gives  a somewhat  better  combination. 

The  combination  greenish-yellow  and  orange-yellow  is 
improved  by  darkening  the  latter  colour,  which  then  appears 
brownish  (R.)*  Grreenish-yellow  and  cyan-blue  make  a bet- 
ter combination  when  the  blue  is  darkened  (R.). 

Grass-green  with  violet  gives  good  but  difficult  combinations. 

Grass-green  with  purple-violet  gives  good  but  difficult  combinations. 

Grass-green  with  rose  gives  combinations  of  doubtful  value. 

Grass-green  with  carmine  gives  combinations  of  doubtful  value. 

Grass-green  with  pink  gives  combinations  of  doubtful  value. 

Grass-green  with  blue  gives  combinations  of  doubtful  value. 


The  value  of  the  last  four  combinations  is  a disputed 
matter. . The  combination  green  and  carmine  is  improved 
by  darkening  both  colours  considerably  (R.).  The  combi- 
nation green  and  blue  becomes  better  as  the  green  inclines 
to  yellow  and  the  blue  to  violet  (R.).  The  combination 
green  and  violet,  according  to  Chevreul,  is  better  when  the 
paler  hues  of  these  colours  are  employed. 

Emerald-green  with  violet  gives  strong  but  hard  combinations. 

Emerald-green  with  purple  gives  strong  but  hard  combinations. 

Emerald-green  with  red  gives  strong  but  hard  combinations. 

Emerald-green  with  orange  gives  strong  but  hard  combinations. 

Emerald-green  with  yellow  gives  bad  combinations. 

All  these  combinations  are  very  difficult  to  handle. 
Emerald-green  and  yellow,  when  both  are  much  darkened, 
furnish  somewhat  better  cornbinations  (R.). 

Sea-green  with  vermilion  gives  good  combinations. 

Sea-green  with  red  lead  gives  good  combinations. 

Sea-green  with  violet  gives  good  combinations. 

Sea-green  with  purple-violet  gives  tolerably  good  combinations. 

Sea-green  with  purple-red  gives,  simply  as  pairs,  poor  combinations. 


290 


MODERN  CHROMATICS. 


Sea-green  with  carmine  gives,  simply  as  pairs,  poor  combinations. 
Sea-green  with  blue  gives  bad  combinations. 

Sea-green  with  yellow  gives  bad  combinations. 

The  surface  of  the  green  should  be  much  larger  than 
that  of  the  vermilion  or  red  lead. 

Cyan-blue  with  chrome-yellow  gives  moderate  combinations. 

Cyan-blue  with  Naples-yellow  gives  good  combinations. 

Cyan-blue  with  straw-yellow  gives  good  combinations. 

Cyan-blue  with  carmine  (light  tones)  gives  good  combinations. 
Cyan-blue  with  violet  gives  poor  combinations. 

Cyan-blue  with  purple-violet  gives  poor  combinations. 

Cyan-blue  with  ultramarine  gives  good  combinations  (small  interval). 

The  combinations  of  cyan-blue  with  violet  and  purple- 
violet  are  not  good,  except  in  fine  materials  and  light 
tones. 

Ultramarine  with  carmine  gives  poorer  combinations  than  cyan-blue. 
Ultramarine  with  purple-red  gives  poorer  combinations  than  cyan-blue. 
Ultramarine  with  violet  gives,  simply  as  pairs,  poor  combinations. 

Violet  with  purple  gives  poor  combinations  if  extended  beyond  the 
small  interval. 

Violet  with  carmine  gives  poor  combinations. 


In  studying  the  effects  produced  by  colours  in  combina- 
tion, it  is  of  course  important  to  exclude  as  far  as  possible 
all  extraneous  causes  that  might  influence  or  confuse  the 
judgment.  Hence  the  colours  under  examination  should  be 
disjtosed  in  very  simple  patterns,  as  the  employment  of 
beautiful  form  or  good  composition  might  easily  become  a 
means  of  leading  the  student  to  accept,  as  good,  combina- 
tions that  owed  their  beauty  to  something  besides  mere 
colour.  For  the  same  reason,  gradation  and  good  light- 
and-shade  effect  should  in  such  examinations  be  avoided ; 
for  these,  as  well  as  good  composition,  are  means  of  con- 
cealing to  some  extent  the  poverty  of  a colour-combination. 
For  a similar  reason  the  materials  employed  in  such  experi- 


COMBINATION  OF  COLOURS  IN  PAIRS  AND  TRIADS.  291 


ments  should  not  be  too  fine.  Almost  any  colour-combina- 
tion worked  out  in  stained  glass  appears  pretty  good,  owing 
to  the  brilliancy  of  the  coloured  light.  This  is  one  reason 
why  the  patterns  in  a kaleidoscope  have  been  of  so  little 
value  in  decorative  art  ; for,  when  the  colours  are  most 
carefully  imitated  in  coarser  materials,  they  are  apt  not 
only  to  lose  their  brilliancy,  but  even  sometimes  to  appear 
dull  or  dirty  from  the  effects  of  harmful  contrast,  which 
did  not  make  itself  felt  before.  To  a less  degree  this 
applies  also  to  silk  ; many  colour-combinations  worked  out 
in  this  material  are  tolerable  on  account  of  its  high  reflect- 
ing power,  while  the  same  colours,  if  transferred  to  wool  or 
cotton,  appear  poor  enough. 

In  forming  a judgment  as  to  the  value  of  combinations 
of  colour,  we  should  also  be  cautious  in  basing  our  conclu- 
sions even  on  observations  made  directly  from  nature  itself  ; 
for  here  our  judgment  is  liable  to  be  warped  by  the  pres- 
ence of  beautiful  form,  good  composition,  exquisite  grada- 
tion, and  high  luminosity.  Green  and  blue,  for  example, 
make  a poor  combination,  and  yet  it  is  one  constantly 
occurring  in  nature,  as  in  the  case  where  the  blue  sky  is 
seen  through  green  foliage.  This  effect  is  often  very  good, 
but  a careful  examination  will  show  that,  in  most  cases, 
blue  and  green  do  not  really  come  in  contact  ; for  if  the 
sunlight  penetrates  the  leaves  in  contact  with  the  sky,  they 
no  longer  look  green,  but  greenish-yellow,  and  this  colour 
makes  a tolerable  combination,  particularly  with  ultramarine- 
blue.  Generally,  however,  the  leaves  actually  in  contact 
with  the  sky  are  in  the  shade,  or  at  least  do  not  send  bright 
light  to  the  eye,  and  we  have  really  greenish-grey  or 
brownish-green  combined  with  the  blue  of  the  sky.  When 
green  actually  does  fairly  touch  the  blue  of  the  sky,  as 
with  a forest  of  young  trees  growing  thickly,  the  green  is 
usually  far  darker  than  the  blue  sky,  as  may  be  seen  by 
closing  the  eyes  partially.  Here  the  combination  is  helped 
somewhat  by  light-and-shade  contrast  ; but  when,  owing  to 


292 


MODERN  CHROMATICS. 


any  cause,  the  blue  of  the  sky  is  darkened  till  it  aj^proxi- 
mates  in  luminosity  to  the  green  of  the  foliage,  then  the 
colour-combination  is  felt  by  an  artist  to  be  bad.  The  forms 
of  trees  are  so  beautiful,  the  variety,  the  gradation  they 
exhibit  so  endless,  the  associations  called  up  by  them  so 
agreeable,  that  we  are  apt  to  deceive  ourselves  about  this 
colour-combination  ; but  when  an  attempt  is  made  to  trans- 
fer it  to  canvas,  we  become  painfully  sensible  of  the  fact 
that  Nature  sometimes  delights  in  working  out  beautiful 
elfects  with  colours  that  are  of  very  doubtful  value,  cun- 
ningly hiding  their  poverty  with  devices  that  often  are  not 
easy  to  discover  or  to  imitate. 

There  are  several  causes  that  may  render  a combination 
of  two  colours  bad.  Prominent  among  them  we  find  the 
matter  of  contrast : the  colours  may  look  dull  and  poor  on 
account  of  harmful  contrast,  or  may  on  the  other  hand 
appear  hard  and  harsh  from  an  excess  of  hel])ful  contrast. 
The  author  has  placed  in  the  form  of  a diagram  the  results 
of  his  observations  on  the  effects  of  contrast  in  diminishing 
or  increasing  the  saturation  or  brilliancy  of  colours.  This 
diagram  (Fig.  131)  and  its  use  are  explained  in  Chapter 
XV.,  and  at  present  we  merely  remind  the  reader  that 
colours  less  than  80°  or  00°  apart  suffer  from  harmful  con- 
trast, while  those  more  distant  helj)  each  other.  In  the 
case  of  colours  that  are  about  80°  apart,  the  matter  remains 
a little  doubtful  ; the  two  colours  may  help  each  other 
somewhat,  or  the  reverse  may  be  true.  On  comparing  this 
diagram  with  the  results  furnished  by  experience  and  given 
in  the  preceding  tables,  it  will  be  found  that  in  good  com- 
binations the  two  colours  are  always  more  than  90°  apart, 
so  that  the  effect  of  contrast  is  mutually  helpful.  Thus, 
red  furnishes  good  combinations  with  blue  and  cyan-blue, 
Avhich  are  considerably  more  than  90°  distant  from  it  ; 
while  the  combination  with  artificial  ultramarine,  which  is 
nearer,  is  inferior,  and  that  with  violet  bad.  It  does  not 
folloAV,  however,  that  the  colours  in  the  diagram  which  are 


COMBINATION  OF  COLOURS  IN  PAIRS  AND  TRIADS.  293 


situated  farthest  apart  always  make  the  best  combinations  ; 
for,  if  this  were  the  case,  the  best  combinations  would  be 
simply  the  complementary  pairs,  which  in  the  diagram  are 
placed  at  the  greatest  distance  from  each  other,  viz.,  oppo- 
site. But  some  of  the  complementary  colours  are  quite 
harsh  from  excessive  contrast  ; for  example,  red  and  its 
complement  green-blue,  also  purple  and  its  complement 
green.  Of  all  the  complementary  pairs,  according  to 
Briicke,  these  are  least  employed  in  art,  as  the  harshness 
with  them  is  at  a maximum.  Now  we  can  divide  the 
colour-diagram.  Fig.  131,  into  two  halves  by  a line  drawn 
from  yellowish-green  to  violet,  and  the  left-hand  half  will 
contain  the  warm  colours,  the  right-hand  the  cold.  After 
doing  this,  we  find  that  red  and  green-blue,  or  purple  and 
green,  are  not  only  complementary,  but  also  situated  at  or 


294 


MODERN  CHROMATICS. 


near  the  positions,  if  we  may  so  express  it,  of  the  greatest 
warmth  and  coldness  ; hence,  owing  to  a double  reason,  the 
contrast  becomes  excessive,  and  the  combination  harsh. 
According  to  the  same  authority,  the  complementaiy  col- 
ours most  in  use  are  ultramarine  and  yellow,  blue  and 
orange-yellow,  or  cyan-blue  and  orange  ; then  follow  violet 
and  greenish-yellow.  In  these  cases,  the  complementary 
pairs  are  situated  at  some  distance  from  the  centres  of 
warmth  and  coldness,  being  in  fact  either  on  or  not  far 
from  the  dividing  line,  which  prevents  excessive  contrast 
and  the  consequent  hardness  complained  of  in  the  examples 
first  cited.  It  may  here  be  remarked  that  colours  which 
are  truly  complementary  often  appear  better  than  those 
which  only  approximate  to  this  condition  ; vermilion  and 
red  lead,  with  their  complements  green-blue  and  greenish- 
blue,  do  not  furnish  sucli  offensive  combinations  as  are 
obtained  when  green  is  substituted  for  the  true  comj^le- 
mentary  hue. 

The  complementary  colours  are  very  valuable  when  the 
artist  is  obliged  to  work  with  dark,  dull,  or  ])ale  colours, 
and  still  is  desirous  of  obtaining  a strong  or  brilliant  effect. 
The  fact  that  the  colours  are  dull  or  pale  or  greyish  pre- 
vents much  possibility  of  harshness  ; and  the  use  of  com- 
plementary hues  excludes  all  risk  of  the  brilliancy  of  the 
tints  being  damaged  by  harmful  contrast.  In  general,  the 
lower  we  go  in  the  scale,  and  the  more  our  colours  approxi- 
mate to  black,  brown,  or  grey,  the  more  freely  can  we 
employ  complementary  hues  without  producing  harshness  ; 
and  even  those  objectionable  pairs,  red  and  green-blue, 
purple  and  green,  if  sufficiently  darkened,  become  agree- 
able. 

It  has  been  stated  above  that  in  good  combinations  the 
colours  are  always  a considerable  distance  apart  in  the 
chromatic  circle.  This,  however,  does  not  exclude  the  class 
of  combinations  mentioned  in  Chapter  XVI.,  where  it  was 
shovTi  that  any  two  colours  differing  but  slightly  produce 


COMBINATION  OF  COLOUBS  IN  PAIRS  AND  TRIADS.  295 


a more  or  less  pleasant  effect ; this  is  the  case  of  the  small 
interval,  which  at  present  we  are  not  considering. 

Now,  although  in  good  combinations  the  colours  are 
rather  far  apart  in  the  chromatic  circle,  it  does  not  follow 
that  all  colours  that  are  far  apart  make  good  combinations. 
When  green,  emerald-green,  or  bluish-green  enters  into  a 
combination,  it  is  apt  to  produce  a harsh  effect  if  the  green 
is  at  all  decided  or  covers  much  space.  The  enormous 
difficulty  of  managing  full  greens  or  bluish-greens  is  per- 
fectly well  understood  by  artists,  and  many  of  them  avoid 
their  use  as  far  as  possible.  The  presence  in  a picture  of  a 
very  moderate  amount  of  a colour  approaching  bluish-green 
or  emerald-green  excites  in  most  persons  a feeling  of  dis- 
gust, and  causes  a work  otherwise  good  to  appear  cold  and 
hard — very  cold  and  hard.  Corresponding  to  this,  most 
artists  seem  to  be  of  the  opinion  that  the  pigment  known  , 
as  emerald-green  is  more  intense  and  saturated  than  any  of 
the  other  colours  used  by  them.  From  a purely  optical 
point  of  view  this  would  seem  hardly  to  be  the  case  : 
emerald-green  reffects  more  white  light  mixed  with  its 
coloured  rays  than  vermilion,  and  its  luminosity  is  not  out 
of  proportion  to  those  of  vermilion  or  ultramarine-blue,  if 
we  adopt  as  our  standard  the  luminosities  of  the  corre- 
sponding colours  in  the  spectrum.  Hence  we  must  seek 
elsewhere  for  the  reason  of  its  unusually  intense  action. 
The  author  is  disposed  to  attribute  this  well-known  intol- 
erance of  all  full  greens  to  the  fact  that  green  light  exhausts 
the  nervous  power  of  the  eye  sooner  than  light  of  any  other 
colour.  This  exhaustion  is  proved  by  the  observation  that 
the  after-pictures,  or  accidental  colours,  are  more  vivid  with 
green  than  with  the  other  colours.  (See  Cliapter  VIII.) 
Now,  as  a general  thing,  very  strong  sensations  are  offensive 
when  freely  interspersed  among  those  that  are  weaker  ; 
thus  Helmholtz  has  shown  that  discord  in  music  is  due  to 
the  presence  of  “ beats,”  which  are  merely  raj)id  alterna- 
tions of  sound  and  silence  following  each  other  at  such 


290 


MODERN  CHROMATICS. 


intervals  as  to  allow  the  sensitiveness  of  the  ear  to  remain 
at  a maximum,  and  hence  producing  disagreeably  intense 
sensations  Avhich  offend.  Quite  analogous  to  this  is  the 
action  of  a flickering  light,  which  is  both  disagreeable  and 
hurtful  to  the  eye.  This  general  principle,  as  it  seems  to 
the  author,  applies  also  to  the  matter  now  in  hand  : a green 
which  optically  may  be  the  equivalent  of  a red,  yellow, 
blue,  or  violet,  nevertheless  produces  on  the  nerves  of  the 
eye  a more  powerful  and  exhausting  sensation  than  these 
colours,  and  hence  is  out  of  harmony  with  them,  or  dis- 
cordant. Besides  this,  and  apart  from  these  considerations, 
green  is  not  a colour  suggestive  of  light  or  warmth,  but  is 
wliat  artists  call  cold  ; the  peculiar  action  above  alluded  to 
renders  it  intense  as  well  as  cold,  and  consequently  painters 
are  only  able  to  employ  it  with  a most  cautious  hand. 
Yellow  conveys  the  idea  of  light,  red  that  of  warmth  : if 
too  much  of  cither  is  present  in  a painting,  the  general 
effect  is  of  course  impaired  ; but  by  a small  over-dose  of 
green,  the  picture  is  killed. 

The  colour  which  next  to  green  acts  niost  powerfully  on 
the  nerves  of  the  eye  is  violet  ; after  that  follows  blue-vio- 
let (artificial  ultramarine-blue).  It  so  happens  that,  among 
the  pigments  at  the  disposal  of  the  painter  or  decorator, 
violet  has  only  a set  of  dull  representatives  ; hence  it  is  not 
quite  so  easy  to  transgress  in  this  direction  as  with  green, 
for  to  obtain  a violet  which  is  at  all  the  0]3tical  equivalent 
of  vermilion  or  emerald-green,  it  is  necessary  to  use  some 
of  the  aniline  colours.  Blue-violet  or  artificial  ultramarine- 
blue  easily  gives  rise  to  cold  and  hard  combinations,  and 
large  surfaces  of  it  are  apt  to  appear  disagreeable  if  the  hue 
is  at  all  intense.  Skies  painted  with  blues  that  are  too  in- 
tense are  easily  ruined,  and  misjudgments  in  this  direction 
are  not  entirely  confined  to  the  work  of  beginners  or  ama- 
teurs. 

When  the  colours  are  arranged  according  to  the  order 
in  which  they  exhaust  the  nervous  power  of  the  eye,  it  is 


COMBINATION  OF  COLOUKS  IN  PAIRS  AND  TRIADS.  297 

found  that  green  heads  the  list ; violet,  blue-violet,  and 
blue  follow ; then  come  red  and  orange,  and  last  of  all  yel- 
low. This  is  also  about  the  order  in  which  we  are  able  to 
enjoy  (or  tolerate)  positive  colour  in  a painting ; large 
masses  of  yellowish  hues  being  often  recognized  only  as 
communicating  luminosity,  while  hues  of  orange  or  red- 
orange,  darkened,  are  called  b;rown,  and  considered  as 
scarcely  more  positive  than  warm  greys.  From  this  it  by 
no  means  follows  that  the  introduction  of  large  masses  of 
positive  green  into  paintings  is  always  to  be  avoided  ; it  is 
not  advisable,  unless  it  can  be  accomplished  successfully^ 
and  without  injury  to  the  work  as  a chromatic  composition. 
The  ability  to  solve  this  problem  in  a brilliant  manner  is 
one  of  the  signs  which  indicate  an  accomplished  colourist ; 
and,  when  the  green  is  combined  with  blue,  the  task  be- 
comes still  more  difficult  and  success  more  praiseworthy. 
On  the  other  hand,  the  handling  of  combinations  of  dull 
yellow,  brown,  grey,  or  bluish-grey  is  much  easier,  and,  in 
fact,  constitutes  the  first  step  by  which  beginners  should 
approach  more  positive  colour. 

As  stated  above,  hurtful  contrast  is  one  of  the  common- 
est reasons  that  render  combinations  of  colour  bad  : as  ex- 
amples, we  have  orange  and  carmine,  yellow  and  yellowish- 
green,  green  and  cyan-blue.  All  the  colours  in  the  con- 
trast-diagram, Fig.  IBl,  that  are  less  than  80°  or  90°  apart 
are  more  or  less  under  the  dominion  of  harmful  contrast. 
These  effects  become  still  more  pronounced  when  the  col- 
ours have  luminosities  decidedly  differing  from  those  found 
in  the  spectrum,  where  yellow  is  the  brightest  and  violet 
the  darkest.  (See  Chapter  III.)  There  are  various  modes 
of  mitigating  to  a considerable  extent  the  effects  of  hurtful 
contrast  : a common  one  is  to  make  one  of  the  contending 
colours  darker  than  its  rival,  or  to  assign  to  it  a mucli 
smaller  field  ; a third  colour  situated  at  a considerable  dis- 
tance in  the  chromatic  circle  is  also  sometimes  added. 
Thus,  for  example,  yellow  and  yellowish-green  are  im- 


298 


MODERN  CHROMATICS. 


proved  by  adding  to  the  combination  a small  quantity  of 
violet  or  purplish-violet  ; green  and  cyan-blue  in  the  same 
way  are  helped  by  the  addition  of  purple  or  orange. 
Harmful  contrast  in  the  matter  of  colour  may  also  some- 
times be  concealed  by  strong  light-and-shade  elfect,  or  by  a 
large  amount  of  gradation,  which  tends  to  enable  all  col- 
ours to  maintain  themselves  against  its  influences  ; beauty 
and  variety  of  forai  also  to  some  extent  mask  its  effects.  It 
may  be  added  that  apparent  truth  to  nature  sometimes 
causes  harmful  contrast  to  be  overlooked  or  pardoned  ; on 
the  other  hand,  soiled  or  impure-looking  tints,  contradiction 
of  nature  either  in  colour  or  form,  and  indecision  of  hand- 
ling, all  are  causes  that  intensify  its  action. 

A combination  may  also  be  poor  because  the  actual  in- 
tensities of  tlie  two  colours  differ  too  much,  although  their 
position  in  the  chromatic  circle  is  advantageous  ; thus,  for 
example,  the  introduction  of  a quantity  of  chrome-yellow 
into  a design  produces  harsh  effects,  which  would  be  avoid- 
ed by  the  use  of  the  more  modest  yellow  ochre.  "When 
this  trouble  exists  in  a high  degree,  the  offending  colour 
usually  catches  the  eye  of  the  observer  at  first  glance,  and 
before  any  of  the  other  colours  are  fairly  seen.  A delicate 
colour-emphasis  is  by  no  means  easy  of  attainment,  and  its 
lack  produces  on  a chromatic  composition  effects  quite  analo- 
gous to  the  want  of  the  corresponding  quality  in  speaking 
or  reading. 

A combination  may  also  be  poor  because  it  contains  no 
decided  representative  of  the  warm  colours,  including  under 
this  term  yellow  and  purple  and  the  colours  situated  be- 
tween them.  There  is  reason  to  believe  that  the  wann  col- 
ours actually  preponderate  in  the  most  attractive  and  bril- 
liant chromatic  compositions  ; however  this  may  be,  it  is 
certain  that  compositions  founded  almost  exclusively  on 
the  colder  colours,  such  as  yellowish-green,  green,  blue,  and 
violet,  appear  poor,  and  are  apt  to  arouse  in  the  mind  of 
the  beholder  a feeling  of  more  or  less  dissatisfaction.  The 


COMBINATION  OF  COLOURS  IN  PAIRS  AND  TRIADS.  299 


general  preference  for  warm  colour  is  somewhat  analogous 
to  that  displayed  for  articles  of  food  that  have  a tendency 
rather  to  sweetness  than  the  reverse  ; but,  however  inter- 
esting an  inquiry  as  to  the  causes  which  during  past  ages 
have  brought  about  this  result  might  be,  it  evidently  would 
not  help  us  much  in  our  present  studies  ; we  are  obliged  to 
accept  the  fact,  and  make  as  good  use  of  it  as  our  skill  and 
feeling  for  colour  permit. 

Thus  far  we  have  considered  the  effects  that  are  pro- 
duced when  the  colours  are  used  in  pairs  ; they  may,  how- 
ever, also  be  employed  in  triads.  The  studies  that  we  have 
made  with  the  contrast-diagram,  Fig.  130,  render  it  easy 
for  us  to  select  a series  of  triads  that  are  free  from  the  de- 
fect of  hurtful  contrast  ; for  this  will  be  the  case  with  all 
colours  that  are  equally  distant  from  each  other  in  the  dia- 
gram, or  are  separated  by  an  angle  of  120°  ; and,  when  we 
examine  the  triads  that  have  been  most  employed  by  artists 
and  decorators,  we  find  that  this  principle  has  actually  been 
more  or  less  closely  observed.  The  triads  that  have  been 
most  extensively  used  are  : 

Spectral  red,  yellow,  blue  ; 

Purple-red,  yellow,  cyan -blue  ; 

Orange,  green,  violet ; 

Orange,  green,  purple-violet. 

In  the  second  triad  the  colours  are  almost  exactly  120° 
apart ; in  the  first  the  yellow  is  a little  less  than  90°  from 
the  red,  and  in  fact  forms  with  it  a doubtful  combination, 
which  is  only  rendered  good  by  the  presence  of  the  blue. 
In  the  third  triad  the  orange  and  violet  are  about  90°  apart, 
but  are  nearly  equally  distant  from  the  green,  and  form, 
both  of  them,  a good  combination  with  it. 

In  the  selection  of  colours  for  these  triads  a second  prin- 
ciple also  seems  to  have  guided  the  choice  of  artists  : there 
is  an  evident  wish  in  each  case  tliat  two  out  of  the  three 
should  be  vmrm  colours,  and  in  two  of  the  triads  the  matter 


300 


MODERN  CHROMATICS. 


of  contrast  has  been  somewhat  sacrificed  for  the  further- 
ance of  this  end.  The  desire  to  satisfy  both  these  condi- 
tions of  course  greatly  limits  the  number  of  triads,  as  an 
examination  of  the  contrast-diagram  shows  ; and,  in  point 
of  fact,  in  certain  inferior  triads  which  have  been  employed, 
one  or  both  of  these  principles  have  necessarily  to  a consid- 
erable extent  been  neglected. 

Carmine,  yellow,  and  green 

was,  according  to  Briicke,  a triad  much  used  during  the 
middle  ages,  though  to  us  the  combination  is  apt  to  appear 
somewhat  hard  and  unrefined.  Here  we  have  two  warm 
colours,  but  the  matter  of  contrast  is  also  twice  sacrificed ; 
that  is,  slightly  in  the  case  of  the  carmine  and  yellow,  and 
more  with  the  yellow  and  green. 

Orange-yellow,  violet,  and  bluisli-green 

is  an  example  of  a combination  which  is  poor  not  from  defect 
of  contrast,  but  because  it  contains  two  cold  colours,  one  of 
them  being  the  coldest  in  the  chromatic  circle. 

Vermilion,  green,  and  violet-blue 

is  a triad  which  has  been  extensively  used  in  some  of  the 
Italian  schools.  At  first  sight  we  have  here  apparently  two 
cold  colours  ; but,  as  the  green  was  olive-green,  the  com- 
bination really  amounts  to 

Vermilion,  dark  greenish-yellow,  and  violet-blue, 

and  corresponds  in  principle  with  those  above  given. 

In  the  employment  of  any  of  these  triads  in  painting  or 
in  ornament,  the  artist  can,  of  course,  vary  the  hue  of  the 
three  colours  through  the  small  interval  without  destroying 
the  definite  character  of  the  chromatic  composition  ; and 
even  small  quantities  of  foreign  colours  can  also  be  added. 
When,  however,  they  begin  to  assume  importance  in  the 
combination,  they  destroy  its  peculiar  character.  White  or 
grey  can  be  introduced,  and  is  often  used  with  a happy 
effect,  particularly  in  the  triads 


COMBINATION  OF  COLOURS  IN  PAIRS  AND  TRIADS.  301 

Orange,  green,  violet ; 

Purple-red,  yellow,  cyan-blue. 

It  is  perhaps  hardly  necessary  to  dwell  on  the  advan- 
tages of  studying  the  relations  of  colours  to  each  other  by 
the  use  of  pairs  and  triads,  before  more  complicated  arrange- 
ments are  attempted.  Many  of  /the  pairs  furnish  opportuni- 
ties for  the  construction  of  beautiful  chromatic  composi- 
tions, and  the  practical  study  of  colour  in  pairs  and  triads 
can  not  be  too  strongly  urged.  In  constructing  a chromatic 
composition,  it  is  also  of  the  first  importance  to  determine 
at  the  outset  what  the  leading  elements  are  to  be  ; after 
this  has  been  done,  it  will  be  comparatively  easy  to  see 
what  variations  are  allowable,  and  what  are  excluded.  The 
most  impressive  and  beautiful  compositions  are  by  no  means 
those  that  contain  the  most  colours  ; far  more  can  be  at- 
tained by  the  use  of  a very  few  colours,  properly  selected, 
varied,  and  repeated  in  different  shades,  from  the  most 
luminous  to  the  darkest. 

We  have  now  examined  to  some  extent  the  good  and  the 
poor  combinations  of  colour,  and  it  may  be  as  well  to  add 
a word  with  regard  to  the  balance  of  colour  ; for  it  is  desir- 
able that  we  should  be  able  not  only  to  select  our  colours 
properly,  but  also  to  provide  them  in  quantities  suitable  for 
the  production  of  the  best  effect.  It  has  been  a common 
opinion  among  English  writers  on  colour,  that  the  best 
result  is  attained  by  arranging  the  relative  areas  of  the 
colours  in  a chromatic  composition  in  such  a way  that  a 
neutral  grey  would  result  if  they  all  were  mixed  together. 
It  is  quite  true  that,  if  the  colours  were  portioned  out  in 
this  manner,  there  would  be  a balance  of  colour  in  an  op- 
tical sense,  though  how  far  balance  in  an  {esthetic  sense 
would  be  attained  is  quite  another  question.  Field  in  his 
“ Chromatics  ” has  given  certain  rules  for  obtaining  an  op- 
tical balance,  anc\  assumes  that  optical  and  {esthetic  balance 
are  one  and  the  same  thing.  For  example,  he  states  that  if 


302 


MODERN  CHROMATICS. 


we  take  red,  yellow,  and  blue,  of  corresponding  intensities, 
then  5 parts  of  red,  3 parts  of  yellow,  and  8 of  blue  will 
neutralize  each  other  in  a mixture,  and  produce  grey  ; also, 
8 parts  of  orange  with  11  of  green  and  13  of  purple  will 
produce  the  same  result  ; likewise  19  parts  of  citrine  (“com- 
pound of  orange  and  green”),  21  parts  of  russet  (“orange 
and  purple  ”),  and  24  parts  of  a mixture  of  olive-green  and 
purple.  These  rules  are  based  on  the  suj^position  that  red, 
yellow,  and  blue  are  fundamental  colour-sensations,  and 
when  mixed  produce  white,  though,  as  we  have  seen  in 
Chapter  IX.,  this  is  quite  the  reverse  of  being  true.  In  a 
mixture  of  red,  yellow,  and  blue,  the  yellow  neutralizes  the 
blue,  since  these  colours  are  complementary,  and  the  super- 
fluous red  strongly  tinges  this  grey  or  Avhite  light,  which 
then  appears  decidedly  reddish.  Field’s  actual  experiments 
on  mixing  colours  were  made  by  transmitting  white  light 
through  hollow  glass  wedges  filled  with  coloured  liquids  ; 
but  it  is,  as  we  have  seen  in  Chapter  X.,  impossible  in  this 
way  to  mix  masses  of  coloured  light.  For  example,  the 
light  which  passes  through  a yellow  and  a blue  wedge  placed 
in  contact  is  merely  that  which  is  not  absorbed  by  either 
wedge,  or  which  both  the  wedges  allow  to  pass.  Both 
wedges  allow  green  light  to  pass,  and  stop  almost  all  the 
other  rays  ; but  from  this  it  is  not  allowable  to  draw,  as 
Field  did,  the  inference  that  yellow  light  and  blue  light 
make  green  light  when  mixed,  since  we  know  with  the  ut- 
most certainty  that  these  two  kinds  of  coloured  light  make 
grey  or  white  light.  Field’s  method  gave  entirely  false  re- 
sults, and  his  conclusions  based  on  them,  including  his  so- 
called  “ chromatic  equivalents,”  have  therefore  for  us  neither 
value  nor  meaning. 

We  return  now  to  the  proposition  that  the  best  effect  is 
produced  when  the  colours  in  a design  are  present  in  such 
proportions  that  a complete  mixture  of  them  would  pro- 
duce a neutral  grey.  It  is  very  easy  with  our  present 
knowledge  to  ascertain  what  areas  we  must  assign  to  two 


COMBINATION  OF  COLOURS  IN  PAIRS  AND  TRIADS.  303 


or  more  coloured  surfaces  in  order  to  realize  this  effect.  It 
is  only  necessary  to  combine,  according  to  Maxwell’s  meth- 
od, rotating  disks  which  are  painted  with  the  pigments  that 
are  to  be  used  in  the  chromatic  composition.  Let  us  exam- 
ine this  matter  with  the  aid  of  a few  actual  examples. 
Taking  the  first  of  the  triads,  spectral  red,  yellow,  and  blue, 
we  find  that  it  is  not  possible  tp  mix  the  colours  in  such 
proportions  as  to  obtain  a neutral  grey  ; the  yellow  and 
blue  neutralize  each  other,  and  the  red  then  colours  the 
mixture  reddish.  The  same  is  true  of  the  triad  carmine, 
green,  yellow  : the  mixture  will  be  orange,  yellowish,  or 
greenish-yellow,  according  to  the  proportions.  In  the  case 
of  the  two  triads,  purple-red,  yellow,  cyan-blue  ; and  orange, 
green,  violet,  neutralization  can  be  produced  by  mixture  ; 
and,  when  the  colours  are  thus  arranged,  the  result  is  more 
pleasing  in  the  first  than  in  the  second  case.  If  we  take 
triads  not  much  used  in  art,  we  meet  with  similar  results  ; 
for  example,  vermilion,  green,  ultramarine-blue,  when  com- 
bined in  such  proportions  as  to  furnish  a grey,  give  a very  un- 
pleasing result,  the  cold  colours  being  greatly  in  excess.  But 
it  is  needless  to  multiply  examples,  as  the  reader  can  easily 
make  these  experiments  for  himself.  If  we  examine  the  areas 
and  intensities  of  the  colours  in  the  works  of  good  colourists, 
we  shall  find  that  they  are  generally  not  such  as  to  produce 
neutrality  when  the  colours  are  mixed  ; that,  on  the  contrary, 
as  in  most  of  the  above  experiments,  there  is  always  an  ex- 
cess of  some  positive  colour.  The  presence  of  this  excess 
gives  a particular  character  to  the  composition,  which  will 
vary  with  the  hue  which  is  thus  emphasized.  Hence  we 
see  that  this  problem  of  the  proper  balance  of  colour  is  one 
which  can  not  be  solved  exactly  by  any  set  of  rules,  but 
must  be  left  to  the  feeling  and  judgment  of  the  artist. 

Attempts  have  been  made  from  time  to  time  to  build 
up  theories  of  colour  based  on  analogies  drawn  from^sound. 
The  sensation  of  sound,  however,  is  more  particularly  con- 
nected with  time,  that  of  sight  with  space  ; and  these  facts 


304 


MODERN  CHROMATICS. 


necessitate  a fundamental  difference  in  tlie  organs  devoted 
to  the  reception  of  sound-waves  and  of  light-waves  ; and, 
on  account  of  this  difference  between  the  eye  and  the  ear, 
all  such  musical  theories  are  quite  worthless.  Thus,  our 
perception  for  colour  does  not  even  extend  over  one  octave, 
while  in  music  seven  octaves  are  employed.  When  two 
musical  sounds  are  mingled,  we  have  accord  or  discord,  and 
the  ear  of  a practised  musician  can  recognize  the  separate 
notes  that  are  struck  ; but,  when  two  masses  of  coloured 
light  are  mingled,  a new  colour  is  produced,  in  which  the 
original  constituents  can  not  be  recognized  even  by  the  eye 
of  a painter.  Thus,  red  and  green  light  when  mixed  fur- 
nish yellow  light  ; and  this  yellow  is  in  no  way  to  be  dis- 
tinguished from  the  yellow  light  of  tiie  spectrum,  except 
that  it  is  somewhat  paler  and  looks  as  though  it  had  been 
mixed  witli  a certain  amount  of  white  light.  Again,  in 
music  the  intervals  are  definite  and  easily  recognized  rela- 
tions, as,  for  example,  that  of  the  fundamental  with  its  fifth 
or  octave  ; we  can  calculate  the  corresponding  intervals  for 
coloured  light,  but  they  can  not  be  accurately  recognized  even 
by  the  most  skillful  painter.  In  painting  we  are  constantly 
obliged  to  advance  from  one  colour  to  another  by  insen- 
sible steps,  but  a proceeding  like  this  in  music  gives  rise  to 
sounds  that  are  ludicrous.  These  facts,  which  are  suscepti- 
ble of  the  most  rigid  proof,  may-suffice  to  show  that  a fun- 
damental difference  exists  between  the  sensations  of  vision 
and  hearing,  and  that  any  theory  of  colour  based  on  our 
musical  experience  must  rest  on  fancy  rather  than  fact. 


CHAPTER  XVIII. 


ON  THE  USE  OF  COLOUR  IN  PAINTING  AND 
DECORATION 

The  power  to  perceive  colour  is  not  one  of  the  most 
indispensable  endowments  of  our  race  ; deprived  of  its  pos- 
session, we  should  be  able  not  only  to  exist,  but  even  to 
attain  a high  state  of  intellectual  and  aesthetic  cultivation. 
Eyes  gifted  merely  with  a sense  for  light  and  shade  would 
answer  quite  well  for  most  practical  purposes,  and  they 
would  still  reveal  to  us  in  the  material  universe  an  amount 
of  beauty  far  transcending  our  capacity  for  reception. 
‘‘  But  over  and  above  this  we  have  received  yet  one  more 
gift,  something  not  quite  necessary,  a benediction  as  it  were, 
in  our  sense  for  and  enjoyment  of  colour.”*  It  is  hardly 
fair  to  say  that  without  this  gift  nature  would  have  ap- 
peared to  us  cold  and  bare  ; still,  we  should  have  lost  the 
enjoyment  of  the  vast  variety  of  pleasant  and  refined  sensa- 
tions produced  by  colour  as  such  and  by  colour-combina- 
tions ; the  magical  drapery  which  is  thus  cast  over  the  vis- 
ible world  would  have  given  place  merely  to  the  simpler 
and  more  logical  gradations  of  light  and  shade.  The  love 
of  colour  is  a part  of  our  constitution  as  much  as  the  love 
of  music  ; it  develops  itself  early  in  childhood,  and  we  see 
it  exhibited  by  savage  as  well  as  cultivated  races.  We  find 
the  love  of  colour  manifesting  and  making  itself  felt  in  the 
strangest  places  ; even  the  most  profound  mathematicians 


* From  an  address  by  Professor  Stcplien  Alexander, 


306 


MODERN  CHROMATICS. 


are  never  weary  of  studying  the  colours  of  polarized  light, 
and  there  can  be  no  doubt  that  the  attractive  power  of  col- 
our has  contributed  largely  to  swell  the  mathematical  liter- 
ature of  this  subject.  The  solar  s^Dectrum  with  its  gorgeous 
tints  was  for  many  years  before  the  discoveries  of  Kirchhoff 
and  Bunsen  a favourite,  almost  a beloved,  subject  of  study 
with  physicists  ; the  great  reward  of  this  devotion  was 
withheld  for  nearly  half  a century  ; divested  of  its  colour- 
charm,  attracting  less  study,  the  spectrum  might  still  have 
remained  an  enigma  for  another  hundred  years. 

Colour  is  less  important  than  form,  but  casts  over  it  a 
peculiar  charm.  If  form  is  wrongly  seen  or  falsely  repre- 
sented, we  feel  as  though  “ the  foundations  were  shaken  ” ; 
if  the  colour  is  bad,  Ave  are  simply  disgusted.  Colour  does 
not  assist  in  de\^eloping  form ; ifr  ornaments  and  at  the 
same  time  slightly  disguises  it  : we  are  content  to  miss 
some  of  the  modeling  of  a beautiful  face  for  the  sake  of 
the  colour-gradations  which  adorn  and  enliven  it. 

The  aims  of  painting  and  decorative  art  are  quite  diver- 
gent, and  as  a logical  consequence  it  results  that  the  use 
made  by  them  of  colour  is  essentially  different.  The  object 
of  painting  is  the  production,  by  the  use  of  colour,  of  more 
or  less  perfect  representations  of  natural  objects.  These 
attempts  are  always  made  in  a serious  spirit  ; that  is,  they 
are  always  accompanied  by  some  earnest  effort  at  realiza- 
tion. If  the  Avork  is  done  directly  from  nature,  and  is  at 
the  same  time  elaborate,  it  will  consist  of  an  attempt  to  repre- 
sent, not  all  the  facts  presented  by  the  scene,  but  only  certain 
classes  of  facts,  namely,  such  as  are  considered  by  the  artist 
most  important  or  most  pictorial,  or  to  harmonize  best  with 
each  other.  If  it  is  a mere  sketch,  it  Avill  include  not 
nearly  so  many  facts  ; and  finally,  if  it  is  merely  a rough 
colour-note,  it  Avill  contain  perhaps  only  a few  suggestions 
belonging  to  a single  class.  But  in  all  this  apparently  care- 
less and  rough  Avork  the  painter  really  deals  with  form, 
light  and  shade,  and  colour,  in  a serious  spirit,  the  conven- 


COLOUR  IN  PAINTING  AND  DECORATION. 


307 


tionalisms  that  are  introduced  being  necessitated  by  lack 
of  time  or  by  choice  of  certain  classes  of  facts  to  the  exclu- 
sion of  others.  The  same  is  true  of  imaginative  painting:  the 
form,  light  and  shade,  and  colour  are  such  as  might  exist  or 
might  be  imagined  to  exist ; our  fundamental  notions  about 
these  matters  are  not  flatly  contradicted.  From  this  it  fol- 
lows that  the  painter  is  to  a considerable  extent  restricted 
in  the  choice  of  his  tints  ; he  must  mainly  use  the  pale  un- 
saturated colours  of  nature,  and  must  often  employ  colour- 
combinations  that  would  be  rejected  by  the  decorator. 
Unlike  the  latter,  he  makes  enormous  use  of  gradation  in 
light  and  shade  and  in  colour  ; labours  to  express  distance, 
and  strives  to  carry  the  eye  beneath  the  surface  of  his  pig- 
ments ; is  delighted  to  hide  as  it  were  his  very  colour,  and 
to  leave  the  observer  in  doubt  as  to  its  nature. 

In  decorative  art,  on  the  other  hand,  the  main  object  is 
to  beautify  a surface  by  the  use  of  colour  rather  than  to 
give  a representation  of  the  facts  of  nature.  Rich  and  in- 
tense colours  are  often  selected,  and  their  effect  is  height- 
ened by  the  free  use  of  gold  and  silver  or  white  and  black ; 
combinations  are  chosen  for  their  beauty  and  effectiveness, 
and  no  serious  effort  is  made  to  lead  the  eye  under  the  sur- 
face. Accurate  representations  of  natural  objects  are 
avoided  ; conventional  substitutes  are  used  ; they  serve  to 
give  variety  and  furnish  an  excuse  for  the  introduction  of 
colour,  which  should  be  beautiful  in  itself  apart  from  any 
reference  to  the  object  represented.  Accurate,  realistic  rep- 
resentations of  natural  objects  mark  the  decline  and  decay 
of  decorative  art.  A painting  is  a representation  of  some- 
thing which  is  not  present  ; an  ornamented  surface  is  essen- 
tially not  a representation  of  a beautiful  absent  object,  but 
is  the  beautiful  object  itself  ; and  we  dislike  to  see  it  for- 
saking its  childlike  independence  and  attempting  at  the 
same  time  both  to  be  and  to  represent  something  beautiful. 
Again,  ornamental  colour  is  used  for  the  production  of  a 
result  which  is  delightful,  while  in  painting  the  aim  of  the 


308 


MODERN  CHROMATICS. 


artist  may  be  to  represent  sorrow,  or  even  a tragic  effect. 
From  all  this  it  follows  that  tlie  ornamenter  enjoys  an 
amount  of  freedom  in  the  original  construction  of  his 
chromatic  composition  which  is  denied  to  the  painter,  who 
is  compelled  by  profession  to  treat  nature  with  at  least  a 
fair  degree  of  seeming  respect.  The  general  structure  of 
the  colour-composition,  however,  being  once  determined,  the 
fancy  and  poetic  feeling  even  of  the  decorator  are  com- 
pelled to  play  within  limits  more  narrow  than  would  be 
supposed  by  the  casual  observer.  It  is  not  artistic  or  sci- 
entific rules  that  hedge  up  the  path,  but  his  own  taste  and 
feeling  for  colour,  and  the  desire  to  obtain  the  best  result 
possible  under  the  given  conditions.  In  point  of  fact,  col- 
our can  only  be  used  successfully  by  those  who  love  it  for 
its  own  sake  apart  from  form,  and  who  have  a distinctly 
developed  colour-talent  or  -faculty  ; training  or  the  obser- 
vance of  rules  will  not  supply  or  conceal  the  absence  of  this 
capacity  in  any  individual  case,  however  much  they  may  do 
for  the  gradual  colour-education  of  the  race. 

From  the  foregoing  it  is  evident  that  the  positions  oc- 
cupied by  colour  in  decoration  and  in  painting  are  essen- 
tially different,  colour  being  used  in  the  latter  primarily  as 
the  means  of  accomplishing  an  end,  while  in  decoration  it 
constitutes  to  a much  greater  degree  the  end  itself.  The 
links  which  connect  decoration  with  painting  are  very 
numerous,  and  the  mode  of  employing  colour  varies  con- 
siderably according  as  we  deal  with  pure  decoration,  or 
with  one  of  the  stages  where  it  begins  to  merge  into 
painting. 

The  simplest  form  of  colour-decoration  is  found  in  those 
cases  where  surfaces  are  enlivened  with  a uniform  layer  of 
colour  for  the  purpose  of  rendering  their  appearance  more 
attractive : thus  woven  stuffs  are  dyed  with  uniform  hues, 
more  or  less  bright  ; buildings  are  painted  with  various 
sober  tints  ; articles  of  furniture  and  their  coverings  are 
treated  in  a similar  manner. 


COLOUR  IN  PAINTING  AND  DECORATION. 


309 


The  use  of  several  colours  upon  the  same  surface  gives 
rise  to  a more  complicated  species  of  ornamentation.  In 
its  very  simplest  form  we  have  merely  hands  of  colour,  or 
geometrical  patterns  made  of  squares,  triangles,  or  hexagons. 
Here  the  artist  has  the  maximum  amount  of  freedom  in  the 
choice  of  colour,  the  surfaces  over  which  it  is  spread  being 
of  the  same  form  and  size,  and  hence  of  the  same  degree 
of  importance.  In  such  cases  the  chromatic  composition 
depends  entirely  on  the  taste  and  fancy  of  the  decorator, 
who  is  much  less  restricted  in  his  selection  than  with  sur- 
faces which  from  the  start  are  unequal  in  size,  and  hence 
vary  in  importance.  After  these  simplest  of  all  patterns 
follow  those  that  are  more  complicated,’  such  as  arabesques, 
fanciful  arrangements  of  straight  and  curved  lines,  or  mere 
suggestions  taken  from  leaves,  flowers,  feathers,  and  other 
objects.  Even  in  these,  the  choice  of  the  colours  is  not 
necessarily  influenced  by  the  actual  colours  of  the  objects 
represented,  but  is  regulated  by  artistic  motives,  so  that  the 
true  colours  of  objects  are  often  replaced  even  by  silver 
or  gold.  Advancing  a step,  we  have  natural  objects,  leaves, 
flowers,  flgures  of  men  or  animals,  used  as  ornaments, 
but  treated  in  a conventional  manner,  some  attention,  how- 
ever, being  paid  to  their  natural  or  local  colours,  as  well  as 
to  their  actual  forms.  In  such  compositions  the  use  of 
gold  or  silver  as  background  or  as  tracery,  also  the  con- 
stant employment  of  contours  more  or  less  decided,  the 
absence  of  shadows,  and  the  frank  disregard  of  local  col- 
our where  it  does  not  suit  the  artist,  all  emphasize  the  fact 
that  nothing  beyond  decoration  is  intended.  Up  to  this 
point  the  artist  is  still  guided  in  his  choice  of  hues  by  the 
wish  of  making  a chromatic  composition  that  shall  be  beau- 
tiful in  its  soft  subdued  tints,  or  brilliant  and  gorgeous  with 
its  rich  display  of  colours  ; hence  intense  and  saturated 
hues  are  often  arranged  in  such  a way  as  to  appear  by  con- 
trast still  more  brilliant ; gold  and  silver,  black  and  white, 
add  to  the  effect ; but  no  attempt  is  made  to  imitate  nature 


310 


MODERN  CHROMATICS. 


in  a realistic  sense.  When,  however,  we  go  some  steps 
further,  and  undertake  to  reproduce  natural  objects  in  a 
serious  spirit,  the  whole  matter  is  entirely  changed  ; when 
we  see  groups  of  flowers  accurately  drawn  in  their  natural 
colours,  correct  representations  of  animals  or  of  the  human 
form,  complete  landscapes  or  views  of  cities,  we  can  he 
certain  that  we  have  left  the  region  of  true  ornamentation 
and  entered  another  which  is  cpiite  different.  A great  part 
of  our  modern  European  decoration  is  really  painting — 
misapplied. 

AVe  return  now  to  a brief  consideration  of  monochromy, 
or  decoration  in  a single  colour.  In  order  to  avoid  the 
monotony  attendant  on  the  use  of  a uniform  surface  of 
colour,  lighter  and  darker  shades  of  the  same  hue  are  very 
often  employed.  These  not  only  give  more  variety,  but 
serve  also  as  a means  of  introducing  various  ornamental 
forms,  such  as  borders,  centre-pieces,  etc.  Monochromy  is 
advantageously  em])loyed  when  it  is  desired,  on  the  one 
hand,  to  avoid  the  brilliancy  attendant  on  the  introduction 
of  several  distinct  colours,  and  on  the  other  the  dullness 
consequent  on  the  exclusive  use  of  a single  tone.  It  is 
much  used  in  wall-painting,  also  in  woven  stuffs  intended 
for  articles  of  dress  or  for  covering  furniture,  and  for  many 
other  purposes. 

In  monochromatic  designs  the  small  interval  is  very 
frequently  employed : for  example,  in  using  red,  the  artist 
will  emj)loy  for  the  lighter  shades  a red  that  is  slightly 
more  orange  than  the  general  ground  ; for  the  darker,  one 
that  is  rather  more  ]mrplish.  In  this  use  of  the  small 
interval,  regard  is  to  be  had  to  the  hues  which  colour 
assumes  under  different  degrees  of  illumination  ; this  mat- 
ter is  fully  explained  in  Chapter  XVII.  Monochromatic 
designs  can  furthermore  be  enlivened  by  ornamenting  them 
with  gold,  either  alone  or  in  connection  with  a small  amount 
of  positive  colour.  The  use  of  black  and  white  is,  however, 
best  avoided,  as  it  furnishes  occasion  for  the  production  of 


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311 


contrast-colours  which  interfere  with  the  general  effect. 
(See  chapter  on  Contrast.) 

In  polychromy  a number  of  distinct  colours  are  em- 
ployed simultaneously,  with  or  without  gold  and  silver, 
white  and  black.  The  laws  which  guide  the  selection  of 
colours  in  this  kind  of  ornamentation  have  already  been 
considered  in  Chapter  XVII.  Saturated  and  intense  colours 
are  often  used  to  cover  only  the  smaller  surfaces  ; they  are 
then  balanced  or  contrasted  with  colours  of  less  intensity 
spread  over  proportionately  larger  surfaces.  In  purely 
decorative  polychromy  we  deal  mainly  with  rich  and  beau- 
tiful arrangements  of  colour  disposed  in  fanciful  forms  ; 
natural  objects,  if  introduced  at  all,  being  treated  conven- 
tionally. In  the  composition  of  such  designs,  however,  the 
artist  is  controlled  to  a considerable  extent  by  the  shape  and 
size  of  the  spaces  which  the  colours  are  destined  to  occupy; 
the  large  masses  of  the  composition  in  the  best  polychromy 
being  worked  out  in  colours  of  proper  intensity,  which 
make  by  themselves  a broad  design,  over  which  again  small- 
er designs  are  wrought  out  in  the  same  and  in  different 
colours.  As  remarked  by  Owen  Jones,*  “The  secret  of 
success  is  the  production  of  a broad  general  effect  by  the 
repetition  of  a few  simple  elements,  variety  being  sought 
rather  in  the  arrangement  of  the  several  portions  of  the 
design  than  in  the  multiplication  of  varied  forms.” 

In  the  best  polychromy  great  use  is  made  of  outlines  or 
contours  ; they  are  employed  to  separate  ornaments  from 
the  ground  on  which  they  are  placed,  particularly  when  the 
two  do  not  differ  greatly  in  colour.  Colours  that  differ 
considerably  are  prevented  by  contours,  on  the  other  hand, 
from  melting  into  each  other  and  thus  giving  rise  to  mix- 
ture tints  ; in  other  words,  each  colour  is  made  by  the 
separating  outline  to  retain  its  proper  position.  Contours 
when  used  for  this  purpose  may  be  light  or  dark  coloured, 

* “Grammar  of  Ornament,”  London,  1866. 

14 


312 


MODERN  CHROMATICS. 


or  even  black.  If  the  ornament  is  lighter  than  the  ground, 
the  contour  is  made  still  lighter ; if  darker,  the  contour 
will  be  still  darker.  In  the  best  decoration  the  figures  of 
men  and  animals,  when  introduced,  are  surrounded  with 
decided  contours  which  emphasize  the  fact  that  realistic 
representation  is  not  intended.  Contours  are  also  made 
white  or  golden  ; they  then  become  an  independent  part  of 
the  ornamentation.  Contours  consisting  of  several  lines  of 
gold  and  silver,  white  and  black,  are  often  used  to  separate 
colours  that  do  not  harmonize  particularly  well  together, 
though,  considered  in  a large  way,  they  may  still  belong  in 
the  compositions.  These  pronounced  contours  are  never 
intended  to  disappear  when  viewed  at  a distance,  but  form 
a new  ornamental  element  ; hence  their  shapes  will  often 
vary  more  or  less  from  the  form  of  the  spaces  which  they 
enclose. 

In  the  richest  polychromy  the  designs  are  mainly  worked 
out  in  intense  or  saturated  colours,  along  with  gold  and 
silver,  white  and  black.  Dark  and  pale  tints  are  not  much 
employed  as  such,  but  are  produced  by  black  or  white 
tracery  on  the  coloured  grounds.  Corresponding  to  this, 
variations  of  the  dominant  colours  are  effected,  not  by  the 
introduction  of  new  tints,  but  by  placing  small  quantities 
of  pure  colour  on  a differently  coloured  ground  ; the  two 
colours  then  blend  on  the  retina  of  the  observer  and  give 
rise  to  the  desired  hue.  For  example,  in  a richly  orna- 
mented table-cover  from  Cairo,  the  writer  noticed  that  the 
use  of  a line  tracery  of  white  on  a blue  ground  gave  rise  to 
the  appearance  of  a lighter  blue,  which  persisted  at  a dis- 
tance ; in  the  border  a pure  red  was  made  to  appear  orange- 
red  by  a tracery  of  yellow  ; in  other  portions,  small  red 
and  white  ornaments  on  a blue  ground  produced  at  a dis- 
tance the  effect  of  a light  violet  tint. 

In  the  superb  decoration  of  the  Alhambra,  the  colours 
employed  on  the  stucco  work  are  red,  blue,  and  gold  ; pur- 
ple, orange,  and  green  are  found  only  in  the  mosaic  dados. 


COLOUR  IN  PAINTING  AND  DECORATION. 


313 


The  colours  are  either  directly  separated  by  narrow  white 
lines,  or  indirectly  by  the  shadows  due  to  the  projecting 
portions  of  the  ornamentation.  Masses  of  colour  are  never 
allowed  to  come  into  contact.  The  blue  and  gold  are  often, 
however,  interwoven  purposely,  so  as  to  produce  at  a dis- 
tance a soft  violet  hue  ; on  this  ground  designs  are  traced 
in  gold  and  red,  the  gold  figures  /being  much  larger  than 
the  red  ; or,  on  the  same  ground  sometimes,  will  be  found 
figures  in  white  with  small  touches  of  red:  The  principle 

for  the  production  of  new  colours  above  mentioned  is  con- 
stantly employed  : blue  and  white  blend  to  a light  blue  ; 
blue,  white,  and  red  furnish  a light  violet  or  purple  hue  ; 
while  red  and  gold  mingle  to  a rich,  subdued  orange. 
Sometimes  in  these  designs  the  gold  greatly  predominates, 
as  in  the  “ Hall  of  the  Ambassadors  ” or  in  the  “ Court  of 
the  Lions  here  we  find  a mass  of  wonderful  gold  tracery, 
with  only  small  portions  of  red  and  blue  imbedded  in  it. 
On  the  dados  the  mosaic  designs  are  often  worked  out  in 
red-purple,  green,  orange-yellow,  and  a dark  blue  of  but 
slight  intensity,  the  ground  being  grey.  Narrow  contours 
of  white  separate  the'  colours  from  the  ground.  To  this 
series  light  blue  is  sometimes  added,  or  we  find  combina- 
tions of  orange-yellow,  dark  blue,  and  green  or  purple  ; 
dark  blue  and  orange-yellow  ; or  simply  orange-yellow  and 
small  spaces  of  dark  blue,  the  grounds  in  all  these  cases 
being  of  a medium  grey.  The  general  effect  of  the  colour 
of  the  mosaics  is  cool  and  somewhat  thin  ; it  rests  the  eye 
which  has  gazed  on  the  magnificent  displays  placed  above, 
or  prepares  it  by  the  contrast  for  new  enjoyment. 

True  polychromy  has  not  been  very  successfully  culti- 
vated in  Europe  since  the  time  of  the  Renaissance,  painting 
having  to  a great  extent  usurped  its  place.  Hence  in  mod- 
ern times  we  find  not  only  our  porcelain,  carpets,  window- 
shades,  but  the  walls  themselves  and  whatever  else  it  may 
be  possible  to  decorate,  covered  with  groups  of  flowers, 
figures,  or  landscapes,  architectural  views,  copies  of  cele- 


314 


MODERN  CHROMATICS. 


brated  paintings — all  executed  with  as  much  pretended 
truth  to  nature  as  the  purchaser  is  able  or  willing  to  re- 
ward. It  is  hardly  necessary  to  add  that  the  taste  which 
produces  or  demands  such  false  decoration,  while  it  may 
have  much  to  excuse,  has  but  little  to  recommend  it ; and 
it  is  not  to  be  expected  that  any  general  improvement  can 
be  elfected  till  the  public  at  large  learns  better  to  distin- 
guish between  genuine  decoration  and  genuine  painting. 

In  decorative  art  the  element  of  colour  is  more  impor- 
tant than  that  of  form  : it  is  essential  that  the  lines  should 
be  graceful  and  show  fancy  or  even  poetic  feeling  ; but  we 
do  not  demand,  or  even  desire,  that  they  should  be  expres- 
sive of  form  in  a realistic  sense.  Just  the  reverse  is  true  in 
painting  : here,  colour  is  subordinate  to  form.  Neverthe- 
less, its  importance  still  remains  very  great,  and  it  is  trifling 
to  attempt  to  adorn  with  colour  that  which  is  really  only  a 
light-and-shade  drawing.  The  chromatic  compositions  of  a 
painting  should  from  the  start  receive  the  most  careful  and 
loving  attention  ; otherwise  it  is  better  to  work  in  simple 
black  and  white. 

The  links  which  connect  designs  in  mere  light  and  shade 
with  works  in  colour  run  about  as  follows  : We  have,  as  the 
first  step,  pictures  executed  essentially  in  one  tint,  but  Avith 
endless  small  modifications.  In  this  way  a peculiar  lumi- 
nous glow  is  introduced  which  is  ncA^er  exhibited  by  designs 
executed  solely  in  black  and  Avhitc,  or  indeed  in  any  one 
tint.  As  examples  of  this  kind  of  AA^ork  we  may  mention 
drawings  in  sepia  or  bistre,  in  which  the  tint  is  Availed  by 
the  introduction  here  and  there  of  different  quantities  of 
some  other  broAAm  haAung  a reddish,  yellowish,  or  orange 
hue.  In  the  next  stage  the  design  is  worked  out  essentially 
in  bluish  and  broAvnish  tints.  If  a landscape,  the  distance 
and  much  of  the  sky  Avill  be  greyish-blue  ; the  foreground, 
on  the  other  hand,  a rich  warm  broAAm,  with  here  and  there 
a few  touches  of  more  positive  color.  The  blue  of  the  dis- 


COLOUR  IN  PAINTING  AND  DECORATION. 


315 


tance  will  be  variously  modified,  having  often  a greenish 
hue,  and  being  replaced  in  the  more  highly  illuminated  por- 
tions by  a yellowish  tint,  hfo  real  attempt  will  be  made  to 
render  correctly  the  natural  colours  of  the  objects  depicted, 
except  as  they  happen  to  fall  in  with  the  system  adopted. 
By  this  mode  of  working,  distance  and  luminosity  can  be 
represented  far  more  effectively  than  by  the  mere  use  of 
black  and  white.  Designs  of  this  kind  merge  by  insensible 
degrees  into  others,  where  the  strong  browns  of  the  fore- 
ground vanish,  and  are  replaced  by  a set  of  tints  which, 
though  not  very  positive,  yet  represent  the  actual  colours 
of  the  scene  somewhat  more  truly.  The  rather  uniform 
bluish-grey  of  the  distance,  also,  is  exchanged  for  a greater 
variety  of  cool  bluish  tints,  and  faint  violet  and  purple  hues 
begin  to  mingle  with  the  other  colours.  The  yellows  and 
orange-yellows  become  more  pronounced,  but  decided  greens 
are  not  admitted  except  in  small  touches,  and  as  the  local 
colour  requires  it  ; large  masses  also  of  any  other  strong 
colours  that  happen  to  be  present  in  the  scene  will  be  sug- 
gested rather  than  represented.  In  designs  of  this  kind 
there  is  a good  deal  of  room  for  the  interchange  and  play  of 
different  hues,  and  they  make  at  first  sight  the  impression 
of  being  veritable  works  in  colour.  Many  of  Turner’s  earlier 
drawings  were  executed  in  accordance  with  these  methods, 
which  allow  the  student  gradually  to  encounter  and  over- 
come the  difficulties  of  colour.  The  substitution  of  paler 
tints  for  the  real  colours  of  the  scene,  and  particularly  the 
exclusion  of  green,  a colour  always  difficult  to  manage, 
diminish  the  possibilities  of  entanglement  in  harsh  or  bad 
combinations  of  colour,  and  render  more  easy  the  attainment 
of  harmony.  This  mode  of  using  colour  is  of  course  con- 
ventional, and  pictures  of  this  kind  are  not  to  be  regarded 
as  executed  in  colour,  in  the  full  sense  of  the  word.  Among 
genuine  works  in  colour,  the  simplest  are  those  painted  es- 
sentially with  a single  pair  of  colours,  variously  modified  or 
combined  with  grey  ; colours  widely  separated  in  tlie  chro- 


316 


MODERN  CHROMATICS. 


matic  circle  from  the  selected  pair  being  admitted  only  in 
small  masses  or  subdued  tones.  After  these  follow  chro- 
matic compositions  in  which  three  colours  with  their  modi- 
fications are  systematically  employed  in  the  same  manner, 
to  the  exclusion  as  far  as  possible  of  all  others.  The  char- 
acter of  these  compositions  will  again  vary  according  as  the 
light  illuminating  the  scene  in  nature  is  supposed  to  be 
white  or  coloured.  If  yellowish,  the  blue  and  violet  hues  will 
be  more  or  less  suppressed,  the  greens  more  yellowish,  while 
the  red,  orange,  and  yellow  tints  will  gain  in  intensity.  Just 
the  reverse  will  occur  under  a bluish  illumination.  The 
practice  of  employing  an  illumination  of  one  dominant 
colour,  which  spreads  itself  over  the  whole  picture,  modify- 
ing all  the  tints,  is  very  common  among  artists,  and  has 
often  been  successfully  used  for  the  production  of  impres- 
sive effects. 

Good  colour  depends  greatly  on  what  may  be  called  the 
chromatic  composition  of  the  picture.  The  plan  for  this 
should  be  most  carefully  considered  and  worked  out  before- 
hand, even  Avith  reference  to  minor  details  ; the  colours 
should  be  selected  and  arranged  so  that  they  all  help  each 
other  cither  by  sympathy  or  by  contrast — so  that  no  one 
could  be  altered  or  spared  Avithout  sensibly  impairing  the 
general  effect.  No  rules  will  enable  a painter  coldly  to 
construct  chromatic  compositions  of  this  character  ; the 
constant  study  of  colour  in  nature  and  in  the  works  of  great 
colourists  AA’ill  do  much,  but  even  more  important  still  is  the 
possession  of  a natural  feeling  for  what  may  be  called  the 
poetry  of  colour,  Avhich  leads  the  artist  almost  instinctively 
to  seize  on  colour-melodies  as  they  occur  in  nature,  and  af- 
terward to  reproduce  them  on  canvas,  with  such  additions 
or  modifications  as  his  feeling  for  colour  impels  him  to  make. 
Thus  it  is  often  adA'isable  to  deepen  nature’s  colours  some- 
what, as  in  the  case  of  the  pale-tinted  greys  of  a distance, 
or  in  the  mere  suggestions  of  colour  often  presented  by 
fiesh.  In  this  process  the  proportions  of  the  coloured  and 


COLOUR  IN  PAINTING  AND  DECORATION. 


317 


white  light  of  nature  are  somewhat  altered,  and  the  col- 
oured element  made  more  prominent.  On  the  other  hand, 
all  the  colours  may  be  made  paler  and  more  greyish  than 
those  of  nature  ; yet  if  they  retain  their  proper  relations, 
if  all  are  correspondingly  affected,  the  harmony  will  not 
be  disturbed,  and  a design  of  this  character  will  still  be, 
from  a chromatic  point  of  view,  logical.  If  the  cold  hues, 
the  greens  and  blues,  are  allowed  to  stand  in  full  strength, 
while  the  warm  colours,  red,  orange,  and  yellow,  are  weak- 
ened, a particularly  bad  effect  is  produced. 

Good  colour,  then,  depends  primarily  on  the  chromatic 
compositions  ; next  in  importance  on  the  drawing,  includ- 
ing under  this  term  outline  and  light  and  shade.  The  want 
of  good,  decided,  and  approximately  accurate  drawing  is 
one  of  the  most  common  causes  that  ruin  the  colour  of 
paintings.  Powerful  drawing  adds  enormously  to  the  value 
of  the  tints  in  a coloured  work  when  they  are  at  all  delicate, 
or  when  the  combination  contains  doubtful  or  poor  colour- 
contrasts,  which  in  point  of  fact  is  a case  common  enough 
in  nature.  Here  the  artist  is  obliged  either  to  reject  the 
material  furnished  by  nature,  or  to  treat  it  in  nature’s  own 
way  ; that  is,  the  drawing  must  be  excellent  and  the  grada- 
tion endless.  Poor  or  bad  combinations  of  colour  are  al- 
most converted  into  good  combinations  by  sufficient  grada- 
tion. When  all  the  tints  are  pale,  as  in  distances,  it  is 
almost  impossible  to  cause  them  to  appear  luminous  or  bril- 
liant without  the  aid  of  delicate  and  accurate  drawing. 
There  is  still  another  way  in  which  the  drawing  influences 
the  colour  : perfectly  clear,  clean  tints  can  be  used,  and  will 
look  well,  where  the  same  colours  in  a slightly  soiled  or  dirty 
condition  would  be  quite  inadmissible.  This  results  from 
the  circumstance  that  helpful  contrast  is  favoured  by  clean, 
even  tints,  while  harmful  contrast  is  strengthened  by  a dirty 
or  spotty  condition  of  the  pigments.  This  is  peculiarly  true 
when  the  colours  are  not  very  positive,  or  are  low  in  the 
scale  ; the  tints,  if  not  clear  and  decided,  instantly  lose  all 


318 


MODERN  CHROMATICS. 


value  and  become  a blemish.  To  insure  this  desirable  ap- 
pearance, called  by  artists  purity,  the  colours  must  be  laid  on 
rapidly  and  with  decision,  and  not  afterward  gradually  cor- 
rected ; but  to  do  this  requires  the  hand  of  an  accomplished 
draughtsman. 

The  advance  from  drawing  to  painting  should  be  grad- 
ual, and  no  serious  attempts  in  colour  should  be  made  till 
the  student  has  attained  undoubted  proficiency  in  outline 
and  in  light  and  shade.  Amateurs  almost  universally 
abandon  black  and  white  for  colour  at  a very  early  stage, 
and  this  circumstance  alone  precludes  all  chance  of  prog- 
ress. The  stage  of  advancement  can,  however,  be  very 
easily  ascertained.  Thus,  for  example,  if  the  student  can 
not  execute  a perfectly  satisfactory  study  of  any  class  of 
subjects  in  outline  with  slight  shade,  then  there  is  no  use 
in  trying  full  light  and  shade  ; if  it  is  impossible  for  him 
to  draw  the  objects  in  full  light  and  shade  in  a rather  mas- 
terly way,  then  there  is  no  use  in  attempting  colour.  The 
method  employed  by  Turner  of  gradually  effecting  the 
transition  from  black  and  white  to  colour  has  been  just 
described,  and  is  worthy  of  the  most  careful  study.  In 
making  the  first  essays  at  colour,  it  is  advantageous  to  exe- 
cute careful  studies  of  the  scene  in  full  light  and  shade, 
but  to  note  down  the  colours  only  in  writing  and  in  the 
memory.  Afterward,  from  these  notes  and  the  black  and 
white  drawing,  a colour-sketch  may  be  attempted,  away 
from  the  scene.  By  this  means  fluctuations  of  judgment 
about  the  colours  and  their  relations  are  avoided,  and, 
though  the  painting  may  be  all  wrong,  it  has  at  least  a 
chance  of  being  executed  on  one  plan,  and  its  frank  errors 
can  afterward  be  ascertained.  Beginners  when  working  in 
the  presence  of  nature  are  apt  to  keep  constantly  altering 
the  plan  of  the  chromatic  composition,  in  the  hope  that  it 
will  at  last  come  right,  and  thus  waste  much  time.  Artists 
under  similar  circumstances  deliberately  make  up  their 
minds  beforehand  what  colour-facts  they  will  take,  what 


COLOUR  IN  PAINTING  AND  DECORATION. 


319 


view  of  the  problem  they  will  adopt,  and  adhere  to  this 
decision  unflinchingly. 

After  some  progress  has  been  made,  the  colour-sketches 
that  are  attempted  directly  from  nature  should  be  simple 
and  executed  with  reference  to  colour,  the  element  of  form 
being  kept  quite  subordinate.  The  very  natural  desire  to 
make  something  that  will  afterward  look  like  a picture  is 
to  be  suppressed,  and  the  work  performed  rather  with  an 
eye  to  the  remote  future.  Beginners  always  neglect  the 
large  relations  of  light  and  shade  and  colour,  dwelling  on 
those  that  are  small  ; whereas  the  aim  of  the  true  artist  is 
the  production  of  a broad  general  effect  by  the  use  of  a 
few  masses  of  colour,  properly  interchanged  and  contrasted, 
variety  being  gained  not  so  much  by  the  introduction  of 
new  colours  as  by  the  repetition  of  the  main  chords.  Va- 
rious modes  of  contending  with  this  evil  have  been  sug- 
gested. One  of  the  simplest  is  making  the  colour-sketches 
so  small  that  there  is  hardly  room  for  anything  but  the 
main  masses  of  colour,  the  use  of  small  brushes  meanwhile 
being  avoided.  Corresponding  to  this,  it  is  frequently 
found  that  if  a picture  by  a beginner  is  actually  cut  up 
into  two  or  more  parts,  the  fragments  thus  produced  are 
better  in  the  matter  of  chromatic  composition  than  the  ori- 
ginal. 

There  are  several  other  stumbling-blocks  that  are  en- 
countered with  much  regularity  by  those  who  make  their 
first  essays  in  colour.  One  of  the  most  important  is  the 
tendency  to  employ  in  the  painting  colours  that  are  vastly 
more  intense  than  those  displayed  by  nature.  The  colours 
of  nature  are  usually  pale  and  low  in  intensity,  even  when 
they  make  upon  the  beholder  just  the  reverse  impression  ; 
and  a practical  knowledge  of  this  fact  is  not  to  be  imme- 
diately attained.  Distant  fields,  for  instance,  often  appear 
to  be  of  a rather  intense  green  hue,  when  the  colour  actu- 
ally presented  to  the  eye  may  be  scarcely  more  than  a grey 
having  in  it  a faint  tinge  of  green.  The  actual  colours 


320 


MODERN  CHROMATICS. 


exhibited  by  different  parts  of  a landscape  may  be  advan- 
tageously studied  by  isolating  them,  according  to  a sugges- 
tion of  Ruskin,  with  the  aid  of  a small  aperture,  half  an 
inch  square,  in  a piece  of  white  cardboard,  held  at  arm’s 
length.  By  this  simple  proceeding  the  student  can  con- 
vince himself  of  the  true  nature  of  the  tints  composing  a 
scene,  for  when  thus  isolated  they  are  not  heightened  by 
contrast.  AYith  such  square  patches  of  colour,  the  judg- 
ment is  not  so  much  affected  by  the  memory  of  the  hues 
which  the  objects  exhibit  at  short  distances,  or  by  what 
artists  call  their  local  colour.  The  local  colour  of  grass  is 
green,  but  if  j)laced  at  a distance  it  may  display  a great 
variety  of  pale  tints,  scarcely  even  greenish  ; yet  owing  to 
the  action  of  the  memory  tlie  distant  grass  still  suggests, 
not  the  idea  of  a variety  of  pale  delicate  greys,  but  of  its 
local  colour,  green.  The  illustration  is  very  old,  but  the 
j)rinciple  applies  riot  only  to  the  greens,  but  to  all  colours  : 
all  will  be  altered  by  distance,  by  the  brightness  of  the  il- 
lumination or  by  its  colour.  The  hues  of  all  objects  are 
also  greatly  affected  by  their  surroundings,  as  explained  in 
the  chapter  on  contrast  ; and  this  is  another  source  of  per- 
plexity and  confusion  to  the  beginner,  who  is  constantly  led 
astray  by  appearances  due  to  this  cause.  The  extent  of  the 
difHculty  can  be  appreciated  when  we  remember  that  con- 
trast affects  not  only  the  intensity  of  the  colour,  but  its 
position  in  the  chromatic  circle,  and  also  its  apparent  lumi- 
nosity, and  is  particularly  lively  in  the  case  of  the  pale 
colours  of  nature.  It  is  as  well  to  meet  this  difficulty  fair- 
ly face  to  face,  and,  instead  of  spending  all  the  disposable 
time  in  endeavoring  to  solve  the  riddles  of  contrast  pre- 
sented by  nature,  to  reverse  the  process,  and  occasionally  to 
construct  in  the  studio  simple  chromatic  compositions  found- 
ed on  the  known  laws  of  contrast,  and  thus  study  its  effects 
by  experiment  as  well  as  by  observation. 

The  appe.arance  of  colour,  as  has  been  explained  in  an- 
other chapter,  depends  also  greatly  on  gradation  ; colour 


COLOUR  IN  PAINTING  AND  DECORATION. 


321 


which  is  uniform  appearing  hard  and  disagreeable,  while 
the  same  tint  when  gently  varied  becomes  pleasing  as  well 
as  truer  to  nature.  Gradation  of  colour  is  almost  universal 
in  nature,  and  a considerable  part  of  the  education  of  the 
student  consists  in  its  study  and  practice.  The  uneducated 
eye  feels  the  effect  of  gradation  in  nature  and  in  a painting, 
but  is  quite  unable  to  trace  thp  delicate  play  of  colour  and 
light  and  shade  on  which  it  depends.  Skill  in  the  use  of 
gradation  gives  the  artist  great  power  to  manage  large 
masses  of  nearly  uniform  colour,  and  an  astonishing  mas- 
tery over  colour-combinations  which  inherently  are  of 
doubtful  value. 

The  enormous  influence  of  good,  decided  drawing  has 
already  been  alluded  to,  but  we  return  again  to  the  matter 
for  a moment,  to  insist  on  the  added  lustre  which  all  the 
tints  of  a painting  acquire  when  connected  with  good,  well 
wrought-out  light-and-shade  effect.  The  beginner  can  most 
easily  convince  himself  of  the  great  influence  which  the 
light-and-shade  effect  exercises  on  the  colour,  by  copying 
the  engraving  of  some  simple  subject  by  a master  in  such 
colours  as  may  seem  most  appropriate,  and  then  comparing 
the  coloring  thus  obtained  with  that  of  his  own  original  de- 
signs. The  selection  of  pigments  in  both  cases  is  by  the 
same  hand,  but  it  will  be  found  that  the  masterly  light  and 
shade  has  given  a value  even  to  the  colours,  which  without 
this  little  plagiarism  tliey  would  not  have  possessed. 

In  painting,  the  selection  of  subjects  on  account  of  their 
chromatic  qualities  is  a very  important  element  of  success. 
It  is  only  by  experience  that  artists  gradually  learn  better 
and  better  how  to  select  their  subjects,  and  the  mistakes  of 
beginners  in  this  respect  are  often  a source  of  prolonged 
discouragement.  Subjects  which  contain  much  green  are 
invariably  difficult  to  manage,  and  should  as  far  as  possible 
be  avoided  in  the  earlier  stages  ; green  fields,  green  trees, 
green  mountains,  all  need  great  skill  if  the  colour  is  ren- 
dered with  any  approach  to  fullness.  This  is  the  reason 


322 


MODERN  CHROMATICS. 


that  the  older  landscapists  lowered  the  colour  of  their  trees 
to  a dull  olive-green,  and  even  to  brown.  Combinations  of 
green,  blue,  and  grey  or  white  are  very  common  in  nature, 
but  difficult  to  handle,  and  necessitate  the  use  of  an  un- 
usual amount  of  gradation.  From  a chromatic  point  of 
view  the  combination  is  poor  ; as  seen  in  nature,  we  do  not 
value  it  less  on  that  account,  perhaps  more.  It  is  delight- 
ful to  see  how  much  can  be  accomplished  wdth  elements  of 
such  doubtful  value.  Again,  effects  which  are  much  de- 
pendent for  beauty  on  very  high  degrees  of  luminosity  are 
difficult,  for  their  pale  colours,  when  transferred  to  canvas 
and  robbed  of  their  natural  luminosity,  are  apt  to  appear 
tame  enough.  In  the  same  category  we  must  place  dis- 
tances, with  their  excessively  pale,  delicate  tints.  The  col- 
ouring in  nature  seems  very  brilliant,  but  in  point  of  fact 
the  effect  is  produced  partly  by  mere  luminosity,  and  partly 
by  the  aid  of  tints  differing  so  slightly  from  each  other  and 
from  grey  that  the  problem  of  imitation,  either  literal  or 
free,  is  not  at  all  easy. 

AVe  might  go  on  in  this  way  adding  to  the  catalogue  of 
art  difficulties,  but  perhaps  it  will  be  asked.  What  subjects 
are  easy?  The  truthful  answer  is  simply,  that  all  are  diffi- 
cult if  even  a moderate  degree  of  excellence  is  demanded. 
The  painter  who  wishes  to  excel  in  colour  devotes  his  life 
to  this  object,  and  is  constantly  accumulating  studies  and 
sketches  from  nature  of  all  kinds  of  subjects,  some  quite 
elaborate,  others  with  less  detail,  many  mere  colour-notes. 
Quite  often  beautiful  effects  of  colour  in  nature  last  only  a 
few  minutes  ; these  will  be  treasured  in  the  memory  and 
transferred  to  paper  or  canvas  the  next  day,  the  sketch 
being  completed  only  far  enough  to  fix  the  facts  in  the 
memory  of  the  artist.  Many  experimental  sketches  will  be 
made,  not  directly  from  nature,  but  with  a view,  as  it  were, 
of  cfuessincr  at  the  elements  on  which  certain  difficult  or 
evanescent  chromatic  effects  depend,  and  also  for  the  pur- 
pose of  ascertaining  their  relations  to  mere  light  and  shade. 


COLOUR  IN  PAINTING  AND  DECORATION. 


323 


This  varied  work  in  colour  will  be  accompanied  by  constant 
practice  in  black  and  white  for  light  and  shade,  and  in  out- 
line for  form,  since  bad  drawing  is  ruinous  to  colour.  All 
the  while  there  will  be  more  or  less  anxious  study  of  the 
works  of  good  colourists,  ancient  and  modern  ; and  this  work 
will  be  pushed  on  month  after  month  with  patient  energy, 
till,  after  a score  of  years  or  sq,  the  student  finally,  if  gifted, 
blossoms  out  into  a colourist. 


NOTE  ON  TWO  RECENT  THEORIES  OF  COLOUR. 


Hering  has  lately  proposed  a theory  of  colour  which  is  quite  different 
from  that  of  Young.  According  to  the  new  theory,  the  retina  is  provided 
with  three  visual  substances,  and  the  fundamental  sensations  are  not  three 
but  six 

Black  and  White. 

' Red  and  Green. 

Blue  and  Yellow. 

Each  of  these  three  pairs  corresponds  to  an  assimilation  or  diassimilation 
process  in  one  of  the  visual  substances ; thus  red  light  acts  on  the  red- 
green  substance  in  exactly  the  opposite  w’ay  from  green  light,  and  wdien 
both  kinds  of  light  are  present  in  suitable  proportions  a balance  is  effected, 
and  both  sensations,  red  and  green,  vanish. 

Furthermore,  according  to  this  theory,  all  the  colours  of  the  spectrum 
also  affect  the  black  and  white  substance  in  the  same  way  that  white  light 
does  ; for  example,  red  light  affects  the  red-green  substance  and  produces 
the  sensation  of  red,  but  it  also  acts  on  the  white-black  substance,  and  the 
sensation  of  red  is  mingled  with  that  of  white — to  a large  degree.  Conse- 
quently, according  to  this  theory,  the  w’hite  which  is  produced  by  mix- 
tures of  red  and  green  light  ought  to  have  a less  intensity  than  the  sum  of 
the  separate  components  ; but  according  to  the  experiments  of  the  author  this 
is  not  the  case.*  For  further  details  the  reader  is  referred  to  the  original 
paper,  “ Lehre  vom  Lichtsinne,”  Vienna,  1878. 

In  1876  F.  Boll  discovered  that  the  retina  contained  a red  or  purple 
substance  that  quickly  disappeared  on  exposure  to  light.  Boll  and  Kiihne 
have  both  studied  the  effect  of  monochromatic  light  on  this  coloured  sub- 
stance, and  it  was  found  that  red  light  intensified  the  hue  at  first  and  after- 
ward caused  it  to  fade  slowly.  The  action  of  yellow  light  was  slow  ; green, 
blue,  and  violet  light  acted  more  quickly.  On  observations  of  this  charac- 


* “ American  Journal  of  Science  and  Arts,”  October,  1877. 


NOTE  ON  TWO  RECENT  THEORIES  OF  COLOUR.  325 


ter  Kiihne  has  constructed  a theory  of  vision.  He  supposes  that  the  waves 
of  light  give  rise  iu  the  retina  to  different  compounds  according  to  their 
length,  and  thus  produce  the  different  colour-sensations.  If  three  such 
compounds  are  thus  produced,  giving  rise  to  the  sensations  red,  green,  and 
violet,  thetf  this  new  theory  is  identical  with  that  of  Young;  if  there  are 
five  such  compounds,  furnishing  the  sensations  red,  yellow,  green,  blue,  vio- 
let, then  the  apparatus  for  yellow  and  blue  has  been  duplicated  in  the 
retina,  since  it  can  be  shown  that  a mixture  of  the  sensations  red  and  green 
gives  that  of  yellow,  a mixture  of  greeil  and  violet  that  of  blue.  Good  rea- 
sons can  also  be  adduced  to  render  probable  the  idea  that  yellow  and  blue 
are  not  fundamental  sensations,  but  mixtures  (compare  the  observations  of 
Bezold  in  Chapter  XII.).  For  additional  information  the  reader  is  referred 
to  the  papers  of  Kiihne  published  in  the  “ Verhandlungen  des  Naturhisto- 
rischo-medicinischen  Vereins  zu  Heidelberg,  18'77-’'T9.” 


INDEX- 


A 

Abnormal  perception  of  colour,  92. 

Absorption,  production  of  colour  by,  65. 

Absorption  and  true  mixture  of  light  com- 
pared, 148. 

Absorption  of  light  by  stained  glass,  65. 

Agassiz,  A.,  observations  of  on  colour  of 
flounders,  101. 

Airy  opposed  to  Brewster’s  theory,  109. 

Albert  on  colour- photography,  b7. 

Alhambra,  decoration  of,  312. 

All  pigments  reflect  some  white  light,  76. 

Antique  glass,  colours  of,  52. 

Aubert  on  mixtures  of  blue  and  white,  196; 
relative  luminosity  of  white  and  black 
paper,  188:  sensitiveness  of  eye  to  mix- 
tures of  white  and  coloured  light,  39. 

B 

Bert,  observations  of  on  chameleon,  101  ; 
on  crustaceans,  11). 

Bezold,  prismatic  colours  change  with  their 
brightness,  181. 

Bierstadt,  e.xperiments  of  in  colour-photog- 
raphy, 87. 

Blake,  Eli,  his  mode  of  recomposing  white 
light,  29. 

Blue,  complement  of,  177. 

Bokowa,  Maria,  artificial  colour-blindness 
of,  97. 

Brewster,  Sir  David,  colour  theory  of,  108 ; 
discoverer  of  cross  and  rings,  47. 

Brucke,  his  apparatus  for  complementary 
colours,  161 ; observations  on  mixtures 
of  blue  and  white,  196, 

Brficke's  solution,  imitates  sky  tints,  1,54. 

Bunsen’s  experiment  on  colour  of  water, 
81. 

C 

Calculation  of  number  of  visible  tints,  40. 

Campbell,  J.,  experiments  on  photographing 
colours  by.  86. 

Chameleon,  its  power  of  imitating  colours, 

101. 

Charts,  colour.  213, 220, 

Chevreul,  colour-chart  of,  222, 

Chrome-yellow,  spectrum  of,  76. 


Colour,  abnormal  perception  of,  92;  appar- 
ent spreading  of  284;  balance  of,  301,  303 ; 
by  moonlight,  187 ; change  of,  with  wave- 
length, 17,  27 ; changed  by  illumination, 
Ibl  ; effect  of  lamp-light  on,  154;  grada- 
tion of,  276;  has  more  than  one  comple- 
ment, 172;  how  affected  by  mingling  it 
with  white,  194 ; is  subjective,  17 ; less 
important  than  form,  306 ; musical  theo- 
ries of,  303  ; of  vegetation,  62 ; of  water, 
81 ; produced  by  absorption,  65;  pro- 
duced by  dispersion,  17 ; produced  by 
electric  current,  9 ; produced  by  opales- 
cent media,  53  ; production  of  by  inter- 
ference, 50 ; relative  luminosity  of  depend- 
ent on  degree  of  illumination,  189 ; repro- 
duction of  by  photography,  86 ; sensation 
of,  produced  by  white  light,  92  ; value  of, 
from  practical  point  of  view,  805. 

Colour  and  wave-length  do  not  change 
equally,  27. 

Colour-blindness,  95,  96 ; of  artists,  100 ; 
means  of  helping,  98;  to  green,  98  ; to 
red,  96. 

Colour-chart,  Chevreul’s,  222  ; of  Du  Fay, 
222 ; of  Le  Blond,  222. 

Colour-charts,  213,  220. 

Colour-combinations,  bad,  292;  bad  owing 
to  absence  of  warm  colours,  298;  bad 
o\ving  to  intensity,  298 ; pairs,  286-299. 

Colour-cone,  216;  and  cylinder,  impossible 
to  execute,  217. 

Colour-contrast,  235-273, 

Colour-cylinders,  215. 

Colour-diagram,  Maxwell's,  224;  Hood’s, 
233. 

Colour-equations,  134. 

Colour-sensations  that  are  not  fundamental, 
115. 

Colour-tbeoiy  of  Brewster.  108  ; of  Touug, 
113;  of  Toung  and  Helmholtz,  113. 

Colon  r-triangle,  221. 

Coloured  light  when  bright  becomes  more 
yellowish,  181-183. 

Coloured  photographs,  indirect  process,  87. 

Coloured  silk  and  wool  compared,  79. 

Colours  can  be  too  pure  and  intense,  SO; 
combined  in  triads.  299-301 : in  combina- 
tion, mode  of  studying,  290 ; fundamental, 
defined,  120 ; in  mixture  represented  by 


INDEX. 


327 


weights,  218;  mixture  of  by  binocular 
vision,  158  ; mixture  of  by  Lambert’s  ap- 
paratus, 139  ; mixture  of  on  retina  of  ob- 
server, 2T9 ; of  metals,  84 ; of  ordinary  ob- 
jects due  to  absorption,  65  ; of  pigments 
due  to  absorption,  65 ; of  prismatic  spec- 
trum, 18;  of  woven  fabrics  due  to  ab- 
sorption, 78 ; photometric,  comparisons  of 
not  absolute,  190 ; prismatic,  change  due 
to  brightness,  181. 

Colours,  complementary,  161 ; explained  by 
Young’s  theory,  176;  by  gas-light,  173 ; 
in  combination,  294;  of  polarized  light 
rather  pale,  177. 

Complementary  colours,  by  gas-light,  173  ; 
explained  by  Young’s  theory,  176;  meth- 
od of  studying  with  Maxwell’s  disks,  167 ; 
luminosity  of,  164;  no  fixed  relation  be- 
tween their  wave-lengths,  175  ; of  polar- 
ized light  are  rather  pale,  177 ; table  of,  163. 

Constants  of  colour,  30--210. 

Contours,  311. 

Contrast,  235-273 ; experiment  with  shad- 
ows, 254  ; hurtful,  297  ; intensity  of  col- 
ours being  different,  263 ; of  black,  white, 
and  grey,  267 ; of  black,  white,  and  grey 
with  colours,  270 ; of  pale  and  dark  col- 
ours, 253-263;  simultaneous,  241-245; 
strength  with  different  colours,  261  ; suc- 
cessive, 235-242 ; table  of  effects  of,  245, 

Contrast-circles,  248. 

Contrast-diagram,  250. 

Cross  and  rings  produced  by  polarized  light, 
47. 

Cross,  C.,  experiments  of  in  colour-photog- 
raphy, 87. 

Curves  for  action  of  red,  green,  and  violet 
on  the  eye,  193. 

D 

Dalton,  colour-blindness  of,  ^7. 

Dalton’s  eye-piece,  36. 

D’Arcy  on  duration  of  impression  on  retina, 
203. 

Dark  lines  of  spectrum,  20, 

Decoration,  different  kinds  of,  309 ; false  aim 
in,  307,  313 ; use  of  one  colour  in,  303, 310; 
use  of  several  colours,  309,  311. 

Decoration  and  painting  divergent  in  aim, 
306. 

De  Haldat  on  binocular  perception  of  colour, 
159. 

Dichrooscope,  Dove’s,  137. 

Diffraction  grating,  23 ; Rutherfurd’s,  ih. 

Diffraction  spectrum,  23. 

Disks,  complementary,  170 ; Maxwell’s, 
109 ; rotating,  used  in  the  study  of  Young’s 
theory,  135. 

Dispersion,  production  of  colour  by,  17. 

Dove  on  binocular  perception  of  colour,  159; 
his  comparison  of  effects  of  absorption  and 
true  mixture  of  light,  143;  dichrooscope 
of,  137 ; his  method  of  studying  comple- 
mentary colours,  165;  observations  on 
relative  luminosity  of  red  and  blue,  189; 
photometric  experiments  on  revolving 
disks,  205 ; theory  of  lu.stre,  280. 

Draper,  II.,  opposed  to  Brewster’s  theory, 
109. 


Drawing,  importance  of,  317,  321. 

Du  Fay,  colour-chart  of,  222. 

Duration  of  impression  on  retina,  202;  in 
case  of  animals  in  motion,  203 ; in  case  of 
ocean  waves,  207. 

E 

Electricity,  production  of  colour  by,  95. 

Emerald-green,  spectrum  of,  75. 

Erythroscope,  83, 

Etchings,  blending  of  white  and  black  on 
retina  of  observer,  282. 

Eye,  colour  of,  58 ; more  sensitive  to  change 
of  wave-length  in  certain  regions  of  the 
spectrum,  27. 

F 

Favre,  examination  of  colour-blind  persons 
by,  99. 

Feathers,  colour  of,  50. 

Fechner  on  colom-s  of  after-images,  93. 

Field,  chromatic  equivalents  of,  301 ; experi- 
ments on  pigments  by,  88,  89. 

Fixed  lines  of  solar  spectrum,  20. 

Fluorescence,  production  of  colour  by,  62. 

Foucault  on  binocular  perception  Of  colour, 
159. 

Fraunhofer,  discovery  of  fixed  lines  by,  20. 

Fundamental  colours  defined,  120 ; intensity 
of,  in  prismatic  spectrum,  123 ; Wunsch 
on,  122. 

Fundamental  colour-sensations,  how  deter- 
mined, 115. 

G 

Gas-light,  effects  of,  on  colours,  154, 

Gibbs,  Wolcott,  on  duration  of  impression 
of  prismatic  colours,  206, 

Glass,  opalescent,  55. 

Glass  under  strain,  colour  of  by  polarized 
light,  48. 

Gold  used  in  pamting,  85. 

Gradation  of  colour,  276;  rapid,  often  un- 
pleasing, 275 ; subordinate  in  decoration, 
283. 

Green  in  colour-combinations,  295. 

Grunow,  William,  spectrometer  of,  21. 

n 

ITarris,  colour-blindness  of,  99. 

Helmholtz  on  colour-blindness,  97 ; colour- 
blind zone  of  normal  eye,  97 ; colours  of 
after-images,  93 ; experiment  of  with  blue 
and  yellow  glass,  188;  on  fundamental 
colours,  120 : mixtures  of  blue  and  yellow, 
190;  mixture  of  prismatic  colours.  111, 
126;  no  fi.xed  relation  exists  between 
wave-lengths  of  complementary  colours, 
175 : prismatic  colours  change  with  their 
brightness,  181. 

Helmholtz  and  Young,  colour  theory  of, 
113. 

Helmholtz’s  spiral  disk  for  after-images,  93. 

Hering’s  theory  of  colour,  324. 

Holmgren,  examination  of  colour-blind  per- 
sons by,  99. 


328 


INDEX. 


Huddart,  remarkable  case  of  colour-blind- 
ness, 99. 

Hue,  36. 

I 

Illumination,  monochromatic,  102. 

Indigo,  complement  of,  178;  its  real  colour, 
21 ; term  improperly  used  by  Newton, 
ih.  ; unfit  designation  of  spectral  hue,  ib. 

Insects,  colour  of  51. 

Interval,  small,  273. 

K 

KTibne’s  theory  of  colour,  324. 

L 

Lambert,  apparatus  of  for  mixing  coloured 
light,  110 ; his  apparatus  used  for  mixing 
colours,  139 ; colour-chart  of,  222. 

Lamp-light,  effects  of  on  colours,  154. 

Le  Blond,  colour-chart  of,  222. 

Listing’s  experiments  on  spectrum,  26. 

Luminosity  of  colour,  83. 

Lustre,  Dove’s  theory  of,  2S0. 

M 

Magnus,  Hugo,  on  development  of  sense  for 
colour,  lOl. 

Maxwell  on  colour-blindness,  103;  funda- 
mental colours,  120;  intensity  of  funda- 
mental colours  in  prismatic  spectrum, 
123;  mixture  of  prismatic  colours,  126 

Maxwell’s  colour-diagram,  224 ; reconstruct- 
ed, 228. 

Maxwell’s  disks,  109,  180- 

Mayer,  A.  M.,  his  historj’  of  Young’s  theor}', 

122. 

Mnver,  II.,  his  arrangements  for  contrast, 
260. 

Mayer,  T.,  co’onr-chart  of,  222. 

Meilium  with  which  pigments  are  mi.xed, 
77. 

Mellonl  opposed  to  Brewster’s  theorj’,  109. 

Metals  used  in  painting,  85. 

Mile,  his  mode  of  mixing  colours,  189. 

Milk,  colours  produced  by,  53. 

Mixture  of  blue  and  yellow  light  makes 
white,  112:  of  colours  by  binocular  vision, 
158:  of  coloured  rays  of  prismatic  spec- 
tnim,  126;  of  different-colonred  light, 
124  ; of  pigments,  theory  and  effects,  141 ; 
of  prismatic  colours.  111;  of  white  and 
coloured  light,  31.  82. 

Monochromatic  illumination,  102. 

Monochromy,  808,  810. 

Moonlight,  colour  of,  187. 

Morton,  H.,  thallene  described  by.  63. 

Mountains,  distant,  colours  of,  59. 

Midler,  .T.  J..  on  fundamental  green.  121 ; 
green  light  in  mixture  produces  a whitish 
tint,  119;  mixture  of  prismatic  colours, 
126. 

N 

Newton’s  diagram  for  the  colour-blind,  105; 
for  lamp-light.  105. 

Newton's  experiment,  18. 


Niepce  de  Saint-Victors  experiments  on 
photographing  colours,  86. 

Nitrate  of  potash,  colours  of,  in  polarized 
light,  47. 

Nobert,  diffraction  grating  of,  23. 

Normal  spectrum,  24,  25;  appearance  of, 

122. 

P 

Painting,  first  practice,  318. 

Painting  and  decoration  divergent  in  aim, 
306. 

Painting  and  drawing,  connecting  links,  814. 

Pettenkofer’s  process,  58. 

Pfaff,  experiment  of  on  optic  nerve  with 
electricity,  9. 

Phosphorescence,  colours  of,  64. 

Photographs,  instantaneous,  peculiarity  of, 
207. 

Photography,  coloured,  thus  far  a failure,  86. 

Pierce,  Charles,  darkened  red  becomes  more 
purplish,  185;  flindamental  green,  120; 
observation  on  colour-blindness,  96 ; pho- 
tometric researches  of,  41. 

Pigments,  action  of  light  on,  88;  appear- 
ance of  affected  by  medium,  77 ; compara- 
tive luminosity  of,  75;  only  three  abso- 
lutely essential,  108 ; peculiar  properties 
of  influence  their  mixtures,  124 ; used  for 
set  of  complementary  disks,  179. 

Pigments  and  stained  glass  compared,  78. 

Pisko,  F.  J.,  on  fluorescence,  68. 

Plateau  on  duration  of  impression  of  col- 
oured light  on  retina,  2U6;  photometric 
experiment  of,  205. 

Plati no-cyanide  of  barium  used  for  fluores- 
cence, 63. 

Polarization,  production  of  colour  by,  43. 

Polarizing  apparatus,  simple,  44. 

Polychromy,  811. 

Pouchet,  observations  of  on  colour  ol  floun- 
ders, 101. 

Preyer  on  colour-blindness,  97,  98. 

Ihismatic  colours,  mode  of  isolating,  19. 

Prismatic  spectrum,  18. 

Purity  of  colour,  32. 

Purkinje,  relative  luminosity  of  w.arm  and 
cold  colours  dependent  oii  the  degree  of 
illumination,  189. 

Purple,  how  produced,  28. 

R 

Ragona  Scina.  apparatus  of  for  contrast,  257. 

Recomposition  of  white  light.  28. 

Red.  sensation  of,  more  intense  when  green 
and  violet  nerves  are  fatigued,  118. 

Red  light,  action  of  on  green  nerves,  117. 

Reflection,  by  polished  surfaces,  11 ; by 
rough  surfaces.  13;  by  water.  12;  of  col-' 
cured  light  by  rough  surfaces,  13 ; of  light, 
11. 

Re?nault  on  binocular  perception  of  colour, 
1.59. 

Retina.  10. 

Rood  on  binocular  perception  of  colour, 
159;  colour-blindness  produced  by  a shock 
to  nervous  svstem,  95 ; coloured  spaces 
in  spectrum,  22,  24;  colours  corresponding 


INDEX. 


329 


to  certain  wave-lengths,  26;  comparison 
of  eifects  of  absorption  and  true  mixture 
of  light,  146 ; complementary  colours  by 
lamp-light,  178;  complementary  disks, 
171 ; complement  of  red,  164 ; contrast- 
circles,  248;  contrast- diagram,  250;  esti- 
mation of  the  coloured  spaces  in  the  pris- 
matic spectrum,  22 ; effects  of  gas-light 
on  colour,  155 ; experiments  on  darkened 
pigments,  185,  188 ; experiments  on  pig- 
ments, 90;  experiments  on  subjective 
colours,  94 ; grey  has  a tendency  to  blue, 
191 ; luminosity  of  pigments,  35 ; method 
of  comparing  the  luminosity  of  white  and 
coloured  surfaces,  34;  mixtures  of  white 
and  coloured  light,  197 ; peculiarity  of 
thin  layers  of  pigments,  199  ; position  of 
pigments  in  normal  spectrum,  38  ; quan- 
titative analysis  of  white  light,  41 ; reflect- 
ing power  of  black  disks,  134 ; reflection 
of  coloured  light  from  coloured  surfaces, 
149;  size  of  coloured  spaces  in  normal 
spectrum,  24;  saturation-diagram,  233; 
time  necessary  for  perception  of  colour, 
102 ; wave-length  corresponding  to  differ- 
ent colours,  26. 

Eutherfurd,  observations  of  on  blue  of  the 
spectrum,  121 ; prismatic  colours  change 
with  their  brightness,  181. 

Eutherfurd’s  automatic  spectroscope,  37  ; 
diffraction  grating,  23,  26  ; diffraction 
plates,  38. 

Euskin  on  mixing  colours,  140;  on  colour- 
gradation,  278. 

S 

Santonin,  colour-blindness  produced  by,  95. 

Saturation-diagram,  Eood’s,  233. 

Schelske  on  colour-blind  zone  of  normal 
eye,  97. 

Seebeck’s  observations  on  colour-blindness, 
96. 

Seguin  on  colours  of  after-images,  93. 

Selenite,  colours  of  in  polarized  light,  44. 

Shadow  confused  with  reflection,  15. 

Shells,  colours  of,  61. 

Simler’s  erythroscope,  83. 

Sky,  colours  of,  58. 

Small  intervals,  table  of,  274. 

Smoke,  opalescent  colours  of,  55. 

Solar  spectrum,  18. 

Soap-bubbles,  colours  of,  49  ; falsely  paint- 
ed, ib. 

Spectra  due  to  chloride  of  chromium,  72. 

Spectrometer,  21. 

Spectroscope,  20. 

Spectrum,  diffraction,  23. 

Spectrum  due  to  blue  glass,  70 ; to  green 
glass,  ih. ; to  green  leaves,  82  ; to  red 
glass,  66 ; to  smalt  paper,  75  ; to  stained 
glass  varies  with  thickness  of  glass,  71. 

Spectrum,  normal,  23  ; appearance  of,  122 ; 
normal  and  prismatic  compared,  23;  of 
orange  glass,  69  ; prismatic,  18. 


Stained  glass,  colour  transmitted  by,  16 ; 
and  pigments  compared,  73. 

Stokes,  researches  of  on  fluorescence,  62. 

Successive  contrast,  235-242. 

Sugar,  colours  of  in  polarized  light,  46. 

Sulphide  of  barium,  phosphorescence  o^  64; 
of  calcium,  ib.  ; of  strontium,  ib. 

Sunset  colours,  normal  series,  61. 

T 

Table  of  fixed  lines  in  solar  spectrum  calcu- 
lated to  1,000  parts,  22. 

'Tables  of  colours  in  pairs,  286-291. 

Tait,  colour-blindness  produced  by  fever,  95. 

Tartaric  acid,  colours  of  in  polarized  light,  46. 

Thallene,  fluorescence  of,  fe. 

Theories  of  colour,  recent,  324. 

Thin  films,  colours  of,  49. 

Translucency,  15. 

Transmission  of  light,  15. 

U 

Uranium  glass,  for  production  of  fluores- 
cence, 62. 

V 

Vegetation  reflects  red  light,  83. 

Veins,  colours  of,  58. 

Velvet,  colours  of,  79. 

Vermilion,  spectrum  of,  75. 

Vierordt’s  photometric  researches  on  the 
spectrum,  33. 

Vision,  theory  of,  11. 

Von  Bezold,  observations  of  on  darkened 
prismatic  spectrum,  183. 

W 

Warm  and  cold  colours,  proportion  of  in 
white  light,  42. 

Wave-length  made  greater  in  fluorescence, 
62. 

Wave-length  and  colour  do  not  chango 
equally,  27. 

Waves  of  light  produce  sensation  of  colour, 
17. 

White  lead  on  dark  ground  appears  bluish, 

66. 

White  light  reflected  from  surfaces  of  pig- 
ments, 76. 

Window-glass,  old,  colours  of,  52. 

Woinow  on  colour-blind  zone  in  normal  eye, 
99. 

Wollaston  noticed  fixed  lines,  20. 

Wiinsch  on  fundamental  colours,  122. 

Y 

Yellow,  complement  of,  177  ; from  a mix- 
ture of  red  and  gi*een  light,  not  very  bril- 
liant, 116. 

Young,  colour  theory  of,  113. 


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“ Professor  Mayer  has  invented  a series  of  experiments  in  Light  which  are 
described  by  Mr.  Barnard.  Nothing  is  more  necessary  for  sound-teaching  than 
experiments  made  by  the  pupil,  and  this  book,  by  considering  the  difficulty  of 
costly  apparatus,  has  rendered  an  important  service  to  teacher  and  student  alike. 
It  deals  with  the  sources  of  light,  reflection,  refraction,  and  decomposition  of 
light.  The  experiments  are  extremely  simple  and  well  suited  to  young  people.” 
—Westminster  Review. 

“This  work  describes,  in  simple  language,  a number  of  experiments  illus- 
trating the  principal  properties  of  light,  by  means  of  a beam  of  sunlight  admitted 
into  a dark  room,  and  various  contrivances.  The  experiments  are  highly  in- 
genious, and  the  young  sPident  can  not  fail  to  learn  a great  deal  from  the  book. 
As  an  example  of  the  effective  experimental  method  employed,  we  may  specially 
mention  the  device  for  illustrating  the  refraction  of  light.  This  book  is  specially 
designed  ‘ to  give  to  every  teacher  and  scholar  the  knowledge  of  the  art  of  experi- 
menting.’ ” — The  (Quarterly  Journal  of  Science  (Loudon). 

“A  singularly  excellent  little  hand-book  for  the  use  of  teachers,  parents,  and 
children.  The  book  is  admirable  both  in  design  and  execution.  The  experi- 
ments for  which  it  provides  are  so  simple  that  an  intelligent  boy  or  girl  can 
easily  make  them,  and  so  beautiful  and  interesting  that  even  the  youngest  chil- 
dren must  enjoy  the  exhibition.  The  experiments  here  described  are  abundantly 
worth  all  that  they  cost  in  money  and  time  in  any  family  where  there  are  boys 
and  girls  to  be  entertained.”— Aew;  York  Evening  Post. 

“The  experiments  are  capitally  selected,  and  equally  as  well  described.  The 
book  is  conspicuously  free  from  the  multiplicity  of  confusing  directions  with 
which  works  of  the  kind  too  often  abound.  There  is  an  abundance  of  excellent 
illustrations.” — New  York  Scientific  American. 

“The  experiments  are  for  the  most  part  new,  and  have  the  merit  of  com- 
bining precision  in  the  methods  with  extreme  simplicity  and  elegance  of  design. 
The  value  of  the  book  is  further  enhanced  by  the  numerous  carefully-drawn  cuts, 
which  add  greatly  to  its  beauty.” — American  Journal  of  Science  and  Arts. 


D.  APPLETON  & CO.,  649  & 661  Broadway,  New  York. 


THE  EXEEEIMEKTAL  SCIENCE  SERIES. 


SOUND: 

A Series  of  Simple ^ Entertaining,  and  Inexpensive  Experiments  iji  the 
Phenomena  of  Sound,  for  the  Use  of  Students  of  Every  Age. 

BY  ALFRED  MARSHALL  MAYER, 

Professor  of  Pliysics  in  the  Stevens  Institute  of  Technology;  Member  of  the 
National  Academy  of  Sciences,  etc. 


Uniform  with  “ LIGHT,”  first  volume  of  the  Series. 


Neat  12mo  volume,  fully  illustr.ted.  . . Cloth,  pkice,  $1.00. 


“The  object  of  the  volume  is  to  present  the  leading  phenomena  of  Sound  in  a 
simple  and  entertaining  manner,  by  the  use  of  such  materials  as  are  almost  every- 
where at  hand,  and  with  apparatus  which  any  ingenious  student  can  construct 
for  himself.  To  present  the  elements  of  an  abstruse  subject  in  such  a way  as  to 
make  the  exposition  easily  comprehensible  by  a mind  not  specially  trained  in 
it,  and  at  the  same  time  correct  and  satisfactory  from  a scientitlc  point  of  view, 
is  one  of  the  most  difficult  undertakings  in  the  v/ork  of  an  instructor.  Add  to 
this  the  task  of  bringing  the  experimental  illustration  of  a science  like  that  of 
acoustics,  which  requires  such  retinement  in  the  apparatus  and  its  manipulation, 
within  the  resources  of  every  one.  and  we  have  the  difficulty  very  greatly  in- 
creased. Professor  Mayer’s  well-known  experimental  skill  has  enabled  him  to 
accomplish  the  work  in  an  admirable  manner,  and  he  has  laid  under  obligation 
to  him  not  only  the  student  and  the  amateur  experimenter,  but  the  teacher,  who 
will  derive  many  valuable  suggestions  as  to  his  own  work  from  this  little  volume. 
The  subject  is  arranged  in  a very  clear  and  methodical  manner,  and  treated  in  a 
vivacious  and  entertaining  rtyle.  The  experiments,  many  of  which  are  novel, 
unite  extreme  simplicity  with  elegance  of  conception  and  scientific  precision, 
and  can  not  fail  to  interest  and  stimulate  the  minds  of  the  students  into  whose 
hands  the  volume  may  fall.  The  illustrations,  which  are  numerous,  are  ex- 
cellently done,  and  give  the  book  a very  attractive  appearance.’’— Awericen  Jour- 
nal^ Science  and  Arts. 

“ It  would  really  be  difficult  to  exaggerate  the  merit,  in  the  sense  of  consum- 
mate adaptation  to  its  modest  end,  of  this  little  treatise  on  ‘ Sound.’  It  teaches 
the  youthful  student  how  to  make  experiments  for  himself,  without  the  help  of 
a trained  operator,  and  at  verv  little  expense.  These  hand-books  of  Professor 
Mayer  should  be  in  the  hands  of  every  teacher  of  the  young.’’— AVifi  lork  Sun. 

“ An  admirably  clear  and  interesting  collection  of  experiments,  described  with 
just  the  right  amount  of  abstract  information  and  no  more,  and  placed  in  pro- 
gressive order.  The  recent  inventions  of  the  phonograph  and  microphone  lend 
an  extraordinary  interest  to  this  whole  field  of  experiment,  which  makes  Pro- 
fessor Mayer's  manual  especially  opporttme.’’— Courier. 


D.  ArPLETON  & CO.,  549  & 651  Broadway,  New  York. 


